Found a Bug in Taskbar rendering?
Looks fine log-out and auto login gives this results.

Will report in thread but no way to give examples but here.
The problem seems to be in Cinnamon Settings>Fonts.
Antialiasing is set to Rgba change to None and problem goes away.
And taskbar normal again. Change it back to Rgba and green square blocks are back.
Don't know if this is a Theme exasperating the problem?
And will boot up fine normal. But log out or reboot then comes back randomly.
Also Troubleshoot Reboot Cinnamon moves all my screenlets to center of my second lcd monitor.
Tho that may be a screenlet issue?
Tried different cinnamon themes losing randomly the Firefox icon left side. Tray side Settings arrow,Volume & All Windows icon invisible. Logging Off or Reboot may or may not restore them. Inconsistent results. Those are consistent icons group that remain invisible. Or restored together when rebooting resolves the issue.
Change Cinnamon Settings Theme other settings changing there and fonts turning on/off Anti-aliasing and hinting. Also switched to TTF corefonts