ACL Failure on Graph API Request to Update Message in Channel

Question

Hi! I've been successfully making GraphAPI requests to update messages in channels using a specific user's delegated permissions for ~10 months. Two weeks ago, this user was briefly deactivated due to inactivity. Once reactivated, I re-created my access token, however exclusively for message updates I get:

APIError Code: 403 message: None error: MainError(additional_data={}, code='Forbidden', details=None, inner_error=InnerError(additional_data={'code': '1', 'message': 'AclCheckFailed-The initiator 8:orgid:c56a34bf-6c6c-4e58-9311-48cad5182570 is not a member of the roster 92:nt_QLzH19t3U1IKSzpCEgvepDsb1oaztI7PbIrkW_Qc1@thread.tacv2 in the generic thread 19:nt_QLzH19t3U1IKSzpCEgvepDsb1oaztI7PbIrkW_Qc1@thread.tacv2'}, client_request_id='caa22ee3-30ed-4afa-be35-93851d9316f5', date=datetime.datetime(2025, 2, 18, 19, 12, 31), odata_type=None, request_id='9367094a-2b36-4db4-80e1-947244f054bb'), message='InsufficientPrivileges', target=None)

I'm cracking my head over it because the user is part of the channel, the token has the permissions, and nothing else seems to be failing; channel creation, message sending (non updating ones that is), and pretty much everything else I do.