I hate the fact that this in a old thread and there has been no solutions or explanations from Apple. The only solution I've seen is to copy, get out of the clip, move the playhead to the start of the clip, delete it, then paste. The problem is when other stuff is done to the clip.
I've been able to repeat the issue in a multicam clip. It was made with Tentacle Sync. (Not saying that's the problem, but just giving some information). In the multicam clip, I made some adjustments due to long recordings with sync issue over time. After a few edits and nudging, I made a compound clip of every camera angle and could not break them apart afterward. I had to undo them. I undid 2 out of the 3, closed FCPX, reopened it and boom. I was no longer able to undo the last camera angle. I had to do the copy paste delete solution.
Although I managed to get through with that solution, I've now got a small seed of doubt in my head when making compound clips on whether or not I can break them apart.
Sorry to chime in on an old thread, but I just wanted to report my findings in case it helps someone else.