Re: #30 and #21. I have the same issue:

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$ for p in /etc/apt/trusted.gpg.d/ubuntu-*; do gpg --no-default-keyring --keyring $p --list-keys; done
/etc/apt/trusted.gpg.d/ubuntu-keyring-2012-cdimage.gpg
------------------------------------------------------
pub   rsa4096 2012-05-11 [SC]
      843938DF228D22F7B3742BC0D94AA3F0EFE21092
uid           [ unknown] Ubuntu CD Image Automatic Signing Key (2012) <email address hidden>

/etc/apt/trusted.gpg.d/ubuntu-keyring-2016-dbgsym.gpg
-----------------------------------------------------
pub   rsa4096 2016-03-21 [SC]
      F2EDC64DC5AEE1F6B9C621F0C8CAB6595FDFF622
uid           [ unknown] Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>

/etc/apt/trusted.gpg.d/ubuntu-keyring-2018-archive.gpg
------------------------------------------------------
pub   rsa4096 2018-09-17 [SC]
      F6ECB3762474EDA9D21B7022871920D1991BC93C
uid           [ unknown] Ubuntu Archive Automatic Signing Key (2018) <email address hidden>

$ sudo apt install --reinstall ubuntu-dbgsym-keyring
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 2 not upgraded.
Need to get 6,904 B of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu groovy-updates/main amd64 ubuntu-dbgsym-keyring all 2020.06.17.3 [6,904 B]
Fetched 6,904 B in 0s (14.4 kB/s)                
Supported
(Reading database ... 704337 files and directories currently installed.)
Preparing to unpack .../ubuntu-dbgsym-keyring_2020.06.17.3_all.deb ...
Unpacking ubuntu-dbgsym-keyring (2020.06.17.3) over (2020.06.17.3) ...
Setting up ubuntu-dbgsym-keyring (2020.06.17.3) ...
$ sudo apt update 
[...]
Ign:28 http://ddebs.ubuntu.com groovy InRelease          
Ign:29 http://ddebs.ubuntu.com groovy-updates InRelease
Hit:30 http://ddebs.ubuntu.com groovy Release
Get:31 http://ddebs.ubuntu.com groovy-updates Release [40.5 kB]
Get:32 http://ddebs.ubuntu.com groovy-updates Release.gpg [819 B]
Err:35 http://ddebs.ubuntu.com groovy Release.gpg
  The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
Err:32 http://ddebs.ubuntu.com groovy-updates Release.gpg
  The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
Fetched 213 kB in 4s (55.8 kB/s)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ddebs.ubuntu.com groovy Release: The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://ddebs.ubuntu.com groovy-updates Release: The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
W: Failed to fetch http://ddebs.ubuntu.com/dists/groovy/Release.gpg  The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
W: Failed to fetch http://ddebs.ubuntu.com/dists/groovy-updates/Release.gpg  The following signatures were invalid: EXPKEYSIG C8CAB6595FDFF622 Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>
W: Some index files failed to download. They have been ignored, or old ones used instead.
$ apt-key export C8CAB6595FDFF622 | gpg --list-packets
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
# off=0 ctb=99 tag=6 hlen=3 plen=525
:public key packet:
        version 4, algo 1, created 1458555883, expires 0
        pkey[0]: [4096 bits]
        pkey[1]: [17 bits]
        keyid: C8CAB6595FDFF622
# off=528 ctb=b4 tag=13 hlen=2 plen=90
:user ID packet: "Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>"
# off=620 ctb=89 tag=2 hlen=3 plen=540
:signature packet: algo 1, keyid D14EF15DAFE11347
        version 4, created 1458556323, md5len 0, sigclass 0x10
        digest algo 8, begin of digest 6f 9e
        hashed subpkt 2 len 4 (sig created 2016-03-21)
        subpkt 16 len 8 (issuer key ID D14EF15DAFE11347)
        data: [4096 bits]
# off=1163 ctb=89 tag=2 hlen=3 plen=540
:signature packet: algo 1, keyid 5759F35001AA4A64
        version 4, created 1496169986, md5len 0, sigclass 0x10
        digest algo 10, begin of digest 23 81
        hashed subpkt 2 len 4 (sig created 2017-05-30)
        subpkt 16 len 8 (issuer key ID 5759F35001AA4A64)
        data: [4095 bits]
# off=1706 ctb=89 tag=2 hlen=3 plen=563
:signature packet: algo 1, keyid 6C39C1C16A9DA7BE
        version 4, created 1535168473, md5len 0, sigclass 0x10
        digest algo 10, begin of digest be 90
        hashed subpkt 33 len 21 (issuer fpr v4 4A90974BACE0A9A6AF09B3B16C39C1C16A9DA7BE)
        hashed subpkt 2 len 4 (sig created 2018-08-25)
        subpkt 16 len 8 (issuer key ID 6C39C1C16A9DA7BE)
        data: [4092 bits]
# off=2272 ctb=89 tag=2 hlen=3 plen=574
:signature packet: algo 1, keyid C8CAB6595FDFF622
        version 4, created 1458555883, md5len 0, sigclass 0x13
        digest algo 2, begin of digest 0e 82
        hashed subpkt 2 len 4 (sig created 2016-03-21)
        hashed subpkt 27 len 1 (key flags: 03)
        hashed subpkt 9 len 4 (key expires after 5y0d0h0m)
        hashed subpkt 11 len 5 (pref-sym-algos: 9 8 7 3 2)
        hashed subpkt 21 len 5 (pref-hash-algos: 8 2 9 10 11)
        hashed subpkt 22 len 3 (pref-zip-algos: 2 3 1)
        hashed subpkt 30 len 1 (features: 01)
        hashed subpkt 23 len 1 (keyserver preferences: 80)
        subpkt 16 len 8 (issuer key ID C8CAB6595FDFF622)
        data: [4096 bits]
# off=2849 ctb=99 tag=6 hlen=3 plen=525
:public key packet:
        version 4, algo 1, created 1458555883, expires 0
        pkey[0]: [4096 bits]
        pkey[1]: [17 bits]
        keyid: C8CAB6595FDFF622
# off=3377 ctb=b4 tag=13 hlen=2 plen=90
:user ID packet: "Ubuntu Debug Symbol Archive Automatic Signing Key (2016) <email address hidden>"
# off=3469 ctb=89 tag=2 hlen=3 plen=540
:signature packet: algo 1, keyid D14EF15DAFE11347
        version 4, created 1458556323, md5len 0, sigclass 0x10
        digest algo 8, begin of digest 6f 9e
        hashed subpkt 2 len 4 (sig created 2016-03-21)
        subpkt 16 len 8 (issuer key ID D14EF15DAFE11347)
        data: [4096 bits]
# off=4012 ctb=89 tag=2 hlen=3 plen=540
:signature packet: algo 1, keyid 5759F35001AA4A64
        version 4, created 1496169986, md5len 0, sigclass 0x10
        digest algo 10, begin of digest 23 81
        hashed subpkt 2 len 4 (sig created 2017-05-30)
        subpkt 16 len 8 (issuer key ID 5759F35001AA4A64)
        data: [4095 bits]
# off=4555 ctb=89 tag=2 hlen=3 plen=568
:signature packet: algo 1, keyid C8CAB6595FDFF622
        version 4, created 1616793053, md5len 0, sigclass 0x13
        digest algo 2, begin of digest 3a 42
        hashed subpkt 27 len 1 (key flags: 03)
        hashed subpkt 11 len 5 (pref-sym-algos: 9 8 7 3 2)
        hashed subpkt 21 len 5 (pref-hash-algos: 8 2 9 10 11)
        hashed subpkt 22 len 3 (pref-zip-algos: 2 3 1)
        hashed subpkt 30 len 1 (features: 01)
        hashed subpkt 23 len 1 (keyserver preferences: 80)
        hashed subpkt 2 len 4 (sig created 2021-03-26)
        subpkt 16 len 8 (issuer key ID C8CAB6595FDFF622)
        data: [4096 bits]
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