I've also now exhausted all lines I thought possible to try (spoke with google, firmware, powerwash, settings, deprovision, custom/other chromeos image) and get round this issue, but it looks like its game over. We have around 200 old Viglen/XMA Chromebook11 devices which, albeit old and 4 years out of updates, still operated. With the age of them, and ones that had no battery replacements, have been great to hand to students to use from home where there was no facilities, all now completely unusable.
I'm still waiting for 'further response' after having a chat to someone in Google Workspace Device support, who unfortunately was very shy on speaking about the issue when I said Im aware they are out of update period. They definitely should have had a fall back where you could at least deprovision the device so that after a reimage or powerwash it could see it was released off the Workspace and work solo.