When I send https://graph.microsoft.com/v1.0/users/<user_id>/mailFolders/<folder_id>/messages/delta api, and use next track link to keep pulling data.
I've found that in a folder Sync Issues->Conflicts, there is a contact item found. (eg. can be listed only via users/<user_id>/contacts api, but not users/<user_id>/messages api).
It's strange that a contact item is in a mail folder. I've also used ews GetItem api to check that it really is a contact item.
Does anyone know how this could happen?