I'm not able to reproduce this issue but I can't restrict my router to use 802.11ac only, it also uses 802.11n in the 5GHz spectrum. Is there a way to check which standard is in use for a connection or is it possible to restrict which standard to use on the client side?

Brian, are you still seeing this issue with a current focal image and kernel 5.4.0-1009?

FWIW, my log:
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd[1]: Starting Network Service...
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd[1]: Started WPA supplicant for netplan wlan0.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: Successfully initialized wpa_supplicant
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-networkd[1593]: eth0: Gained IPv6LL
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-networkd[1593]: Enumeration completed
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Network configuration changed, trying to establish connection.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Network configuration changed, trying to establish connection.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd[1]: Started Network Service.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: IPv6 successfully enabled
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-networkd[1593]: eth0: IPv6 successfully enabled
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Network configuration changed, trying to establish connection.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-networkd[1593]: eth0: DHCPv4 address 192.168.99.34/24 via 192.168.99.1
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Network configuration changed, trying to establish connection.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Network configuration changed, trying to establish connection.
Apr 29 06:17:46 rpi-4b-rev1d1-17cf systemd-timesyncd[1092]: Initial synchronization to time server 192.168.99.1:123 (192.168.99.1).
Apr 29 06:17:46 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:47 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:48 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:49 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:50 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:51 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:52 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SCAN-FAILED ret=-16 retry=1
Apr 29 06:17:53 rpi-4b-rev1d1-17cf kernel: ieee80211 phy0: brcmf_escan_timeout: timer expired
Apr 29 06:17:55 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: Trying to associate with SSID 'pi-test'
Apr 29 06:17:58 rpi-4b-rev1d1-17cf iwd[1176]: Unexpected connection related event -- is another supplicant running?
Apr 29 06:17:58 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: Associated with e0:28:6d:9e:b9:25
Apr 29 06:17:58 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-CONNECTED - Connection to e0:28:6d:9e:b9:25 completed [id=0 id_str=]
Apr 29 06:17:58 rpi-4b-rev1d1-17cf kernel: IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
Apr 29 06:17:58 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SUBNET-STATUS-UPDATE status=0
Apr 29 06:17:58 rpi-4b-rev1d1-17cf systemd-udevd[712]: Network interface NamePolicy= disabled on kernel command line, ignoring.
Apr 29 06:17:58 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: Gained carrier
Apr 29 06:17:58 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: Connected WiFi access point: pi-test (e0:28:6d:9e:b9:25)
Apr 29 06:17:58 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: DHCPv4 address 192.168.99.39/24 via 192.168.99.1
Apr 29 06:18:00 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: Gained IPv6LL
Apr 29 06:19:21 rpi-4b-rev1d1-17cf dbus-daemon[1170]: [system] Activating via systemd: service name='org.freedesktop.PackageKit' unit='packagekit.service' requested by ':1.15' (uid=0 pid=1703 comm="/usr/bin/gdbus call --system --dest org.freedeskto" label="unconfined")
Apr 29 06:19:21 rpi-4b-rev1d1-17cf systemd[1]: Starting PackageKit Daemon...
Apr 29 06:19:21 rpi-4b-rev1d1-17cf PackageKit[1706]: daemon start
Apr 29 06:19:21 rpi-4b-rev1d1-17cf dbus-daemon[1170]: [system] Successfully activated service 'org.freedesktop.PackageKit'
Apr 29 06:19:21 rpi-4b-rev1d1-17cf systemd[1]: Started PackageKit Daemon.
Apr 29 06:24:27 rpi-4b-rev1d1-17cf PackageKit[1706]: daemon quit
Apr 29 06:24:27 rpi-4b-rev1d1-17cf systemd[1]: packagekit.service: Succeeded.
Apr 29 06:25:01 rpi-4b-rev1d1-17cf CRON[1788]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 29 06:25:01 rpi-4b-rev1d1-17cf CRON[1789]: (root) CMD (test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily ))
Apr 29 06:25:02 rpi-4b-rev1d1-17cf CRON[1788]: pam_unix(cron:session): session closed for user root
Apr 29 06:29:57 rpi-4b-rev1d1-17cf systemd[1]: Starting Cleanup of Temporary Directories...
Apr 29 06:29:57 rpi-4b-rev1d1-17cf systemd[1]: systemd-tmpfiles-clean.service: Succeeded.
Apr 29 06:29:57 rpi-4b-rev1d1-17cf systemd[1]: Finished Cleanup of Temporary Directories.
Apr 29 06:31:20 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: Associated with e0:28:6d:9e:b9:25
Apr 29 06:31:20 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-CONNECTED - Connection to e0:28:6d:9e:b9:25 completed [id=0 id_str=]
Apr 29 06:31:20 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: Lost carrier
Apr 29 06:31:20 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: DHCP lease lost
Apr 29 06:31:20 rpi-4b-rev1d1-17cf wpa_supplicant[1598]: wlan0: CTRL-EVENT-SUBNET-STATUS-UPDATE status=0
Apr 29 06:31:20 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: Gained carrier
Apr 29 06:31:20 rpi-4b-rev1d1-17cf systemd-networkd[1593]: wlan0: DHCPv4 address 192.168.99.39/24 via 192.168.99.1