Hello @Graeme Bennett ,
Thanks for your feedback.
Did you try to change the GPP to update instead of replace?
https://www.meinekleinefarm.net/caution-fslogix-2009-2-9-7621-30127-profiles-wont-logoff-completely/
It seems a known issue with fslogix that you can check the following post for reference:
https://learn.microsoft.com/en-us/answers/questions/229309/fslogix-unclean-logoff-causing-locked-files-until.html?page=5&pageSize=10&sort=oldest
The workaround may be reboot the server.
I've synced with our internal team that customer can submit fslogix feature suggestions here and let others upvote for it to see if there will be any feedback from the product team: https://techcommunity.microsoft.com/t5/azure-virtual-desktop/idb-p/AzureVirtualDesktop