multicam thumb nails & viewer showing black on select clips in fcp xi

Double-click the multicam clip to open the Angle Editor. What do you see in that area?

Seeing clips black could just mean that the audio goes on after the video clips had ended, but without seeing the Angle Editor this is just a guess.

On rare occasions when I've had multicam angles go black, opening it in the angle viewer and changing the monitoring angle to a different angle seems to wake it up, though sometimes just temporarily. I wasn't able to find any common threads about why this might happen -- media specs, project specs, and multicam specs were all the standard way we create them.

Your behavior in the angle editor is a little different -- I would see missing/black angles in the angle editor and in the timeline. Changing the monitoring angle in the angle editor would bring the black angles back in the angle editor and then also in the timeline. Your angles in the angle editor all look fine.

What happens if you optimize one of the missing angles? Beware this can make giant ProRes files, but maybe there will be some change in behavior.

Just to confirm, I'm talking about changing active monitoring angle inside the angle viewer. So in your screenshot above, you'd change the monitoring angle from DJI_3570 to one of the other others so the grey shelf encompassed another angle, say DJ2_8182.