I’ve never had an issue with this before, however I haven’t done it since 10.2 so I just tested it out w/ some screen cap clips (pro-ress 422) that I had here on my home machine.
Worked fine.
You might try it out the same way in a test library w/ prores-422 clips. If your using a camera codec and it works with prores then that might indicate a codec issue (or at least you can verify that you’re doing it right).
You do get the option to transcode media if you right-click a multicam clip in the event browser. That might be worth a shot as well.
A couple of other things. You might verify that background rendering is on (in fcpx preferences). Flushing the render files and letting them rebuild also helps with some of the weirder glitches.
As always, there is also the standard apple checklist found here.
https://support.apple.com/en-us/HT203477
Side note:
One thing I just discovered in my quick tests now. Out of curiosity I created a compound clip, thew a generator in that and then tried to make that into a multicam clip. FCPX did not like that at all and I had to force quit.