Multiclip is being really picky right now.
I have the newest final cut version and have a multiclip of 4 cameras. I put the multiclip on the timeline and double clicked to open in the viewer as well. I can see the canvas and the 4 cameras in the viewer when the timeline is paused, when I play the timeline, the viewer changes from 4 cameras to 1 and is identical to whats playing on the canvas

I have multiclip playback checked, and the viewer set to open.

This happens when you have FCP set to autorender while you're away.

The normally RT timeline renders, and then upon playback only plays back one angle. Go into Tools/Render Manager and delete the render file(s) for the affected timeline.

Weird problem to be sure. Took me a while to figure it out first time it happened to me.