Adobe Premiere Pro
The multicam monitor always shows the targeted track in a multicam sequence. So if you have a nested multicam sequence on track 2, target track 2. If it's on track 3, then target track 3, and so on.
I created a new sequence, and then copied / pasted the video & audio from the "defective" sequence. Multi-camera monitor now works as expected, and my old cuts are still there.