When you’re viewing your multicam window, you’re only seeing the single track that the multicam lives on, not the entire timeline. You select this track in the lower center part of that window. So if your multicam is on track one, select track one, track two > select track two etc. Because you are only seeing that single track, the adjustment layer modifications aren’t seen.
If you nest your multicam with an adjustment layer, your multicam window is no longer seeing a multicam in the track you’ve selected – it’s seeing a nested track which it isn’t designed for, and therefore displays nothing/black.
I understand why you might want to see your adjustments during the edit – so you get to cut within the mood/context of the final piece. However, the program isn’t designed to do that > most likely because playing back multiple tracks with applied color correction and effects simultaneously, and doing so all while allowing you to make real time edits, would probably take a spaceship size memory/processor 🙂
Hope this helps.