mate-panel applets are all blank after screen unlock

mate-panel starts normally and seems to work.

WHAT HAPPENS:
But after a while (often after the screen is locked and then unlocked) all the applets go blank and don't work anymore.
I have 2 panels open (a top panel and a bottom panel). Sometimes only the top one is messed up, and sometimes both are messed up. E.g. I am running "clock" in both panels. Sometimes the clock is blank in the top panel, and sometimes it is blank in both panels. Aother applets are also blank (e.g. NetworkManager, blueman-applet, workspace switcher).

The menus seem to always work, even when the other applets are messed up.

The only thing that I've found that fixes it is to restart the panel:
mate-panel --replace

EXPECTED TO HAPPEN:
Applets not be blank.

VERSIONS RUNNING:
> lsb_release -rd
Description: Debian GNU/Linux rodete
Release: rodete

> apt-cache policy mate-panel
mate-panel:
  Installed: 1.24.0-2
  Candidate: 1.24.0-2
  Version table:
 *** 1.24.0-2 600
        600 https://rapture-prod.corp.google.com glinux-rodete-base-20200811.01.07/main amd64 Packages
        100 /var/lib/dpkg/status

> apt-cache 'show' 'mate-panel'
Package: mate-panel
Version: 1.24.0-2
Installed-Size: 1656
Maintainer: Debian+Ubuntu MATE Packaging Team <email address hidden>
Architecture: amd64
...
Filename: pool/mate-panel_1.24.0-2_amd64_90502bb5883dd701cf0365e7357f38afa156ed9faf8ef000ca6c3cc5c864a335.deb
Priority: optional
SHA256: 90502bb5883dd701cf0365e7357f38afa156ed9faf8ef000ca6c3cc5c864a335
Section: x11
Size: 605746

/usr/lib/mate-panel/clock-applet comes from the same package.