Some suggestions you might try: create again a branch from 7.16.0 and mark that branch as to be used by the studio modeler. Double check if your main line is now free of the studio modeler. You can then try the merge again to see if it then works.
Other option would be to do what is stated in this forum post: https://forum.mendix.com/link/questions/8380 but I only would use that one as last resort (note that here they moved main line to branch but the other way around is offcourse also possible with this method).
Regards,
Ronald