Android allows the user to set Google Workspace as a backup option, and will successfully backup. Unfortunately, when I try to restore after a factory reset, I find that I am unable to restore.
The user should not be allowed to backup using Workspace account if they are not going to be able to restore.
I now have two backups on Workspace Drive account without being able to restore either backup.
So the problem about restoring backups after the device is factory reset also happens if the account is a personal Google Account ending with @gmail.com or @googlemail.com so this is not necessarily a unique issue to Workspace if that is what you are trying to imply (users report this sort of thing even on the Android Help Community)..but is the user not getting the option to restore a backup or what exact error message are they getting when they try? And just to confirm, your backups are in Google Workspace and even when you try to set up that exact Google Account on your phone, you are not given an option to restore an Android backup? Is the Android managed by the organization? (Organizations can manage certain types of devices including Chromebooks.)
What type of error message are you getting? What did the Workspace one on one support say when you contacted them via the instructions at https://support.google.com/a/answer/1047213?