UPN invalid for a User When Attempting passwordProfile property Update

Question

I have a hybrid environment with password writeback enabled per Microsoft's documentation. When I attempt to change a synced user's password via Graph API (specifically by setting a new password in their passwordProfile) it keeps telling me that I have an invalid UPN for said user. This is wrong however, as I am able to do other API calls on that UPN just fine. I have done this before in other tenants with no issues so I'm not sure what I am missing here. The response does not give me any further info that that, unfortunately. I've tried this with multiple accounts that are all valid as well in this tenant.