Agree. Push up on the trim. If it lays flat against the ceiling, just put a few brads in the trim. From the picture you should be able to remove the light and see if there's joist on the seam.Welcome to the forum.
Is the whole panel sagging or just the trim which covers the joint? Is there any wooden support above the panels. If it’s just the trim sagging and if there is a wooden joist on top of the panels, just use brads to put it back up and just fill the holes with putty.