Hello,
ArcPro 1.4 does not allow me to edit a many-to-many relationship table. Instead I get an error message saying that the data is locked and in use by another user. There are no other users on my machine. What I close Pro and reopen I get the same error. I don't see any lock files in the geodatabase. 
I realize that Pro Editing sessions are not the same as ArcMap, but I don't have a clue as to how I can fix this. Any ideas? The geodatabase is the default project geodatabase. Maybe that is the problem. Should I not use the geodatabase created for the project by Pro?