I have a mask that applies to an entire video clip - with a few key frames.
For some reason some rendered frames omit the mask completely. After investigating, the output of the mask node in compositing is to mask absolutely nothing.
There are no key frames around the problem frames.
Below is the mask output of frames 9 and 10 in the composite. As you can see 10 masks nothing, where 9 has the 'key hole' mask.