Shared iPads must be supervised. Almost at the bottom of this article:
https://support.apple.com/en-gb/guide/mdm/cad7e2e0cf56/web. "Devices
must have at least 32GB of storage and be supervised."Changing
(un)supervised status requires a full reset.
Have you tried using a Restrictions profile? Or does the upgrade
override the profile (which would be weird)? The profile I'm talking
about can be found: Restrictions > Functionality > somewhat in the
middle of the list > Allow iCloud Desktop & Docum...
