@Jérôme Thank you for reaching out to us, If I understand correctly are you trying to perform user writeback via Cloud Sync? if my understanding of the issue/scenario is incorrect, please let me know. With this configuration Entra ID to AD, if you review the attribute mapping section you will see only groups mapping (which means only group writeback operation is supported from Entra ID to AD).
User writeback is not a supported option. You can sync users/groups/contacts from AD to Entra ID but not the other way. Let me know if you have any further questions, feel free to post back.