When I switch camera angles in multicam edit mode, the previous clip changes

I've tried EVERYTHING and I have no idea what's going on - as this has never happened to me before.

As with all my edits, I upload the video & audio files, drag them into a sequence, syncronize everything, then nest the video files. I then enable the cameras for multicam editing. But today, for some unknown reason, when I switch angles, say from 1 to 3, the previous clip being 1 in this example, also changes to 3. It KEEPS happening.

I've tried scrapping the project and starting fresh - did not work.
I've turned things on and off.
Another thread suggested it was the "selection follows playhead" being on, but turning that off didn't have an impact. 

Please can anyone help? From a very stressed editor who has many many deadlines.