When assigning a Resource Assignment Record:

If the current owner of the record matches who the record is being assigned to, the bookable resource is not updated.

If the current owner of the record does not match who the record is being assigned to, Microsoft updates both the owner of the record and the Bookable Resource.

Please change the code so that when a user assigns a Resource Assignment record, the Bookable resource is also updated every time. This will provide a consistent experience for the users, and increase productivity by saving the user from having to perform the extra steps to get the bookable resource assigned.