A user has full access permissions to a room mailbox or editing permissions to the calendar of a room mailbox. When the user tries to create a meeting request directly in the calendar of the room mailbox, the room mailbox doesn't process the meeting request.
This behavior is by design. The Resource Booking Assistant processes requests that are sent to the room. When a user who has editing permissions creates the meeting directly in the room’s calendar, the room is the actual organizer of the meeting, and there's nothing to process.
For the room to process all meetings, create the meeting in the calendar of the user who wants to book the room (the user who has editing permissions) instead of creating the meeting directly in the calendar of the room mailbox.