M365 Copilot Chat Works for Cloud-Only Students, Not for AD-Synced Accounts

I want to make M365 Copilot Chat accessible for our students (13+), and according to this article, I have prepared everything:

• My tenant is correctly classified as K12.
• The student account has the attribute ageGroup set to NotAdult in Entra ID.

Unfortunately, this only works with student accounts that are cloud-only. When I synchronize a student from the on-premises AD to Entra ID and set the ageGroup attribute to NotAdult in Entra ID, access to Copilot Chat does not work.

Any ideas?