Black frames during export / strange multicam issue

Hi, exporting a 4k prores feature (3840x2160, 1.0, 23.976, Progressive) to 1920x1080 out of Premiere. In one particular scene I keep getting a black spot. The shot during the scene literally goes black. It is a shot that originates in a separate multicam nested sequence. But in our master timeline for the movie it's just the nest obviously. The fix in editing is when I choose multicam mode and just switch cameras, then all of a sudden it comes back and isn't black anymore. But when I export it still is. Any ideas for this very very strange problem?