volume graphs drop out in omf

I exported my 3 audio tracks of my film to omf and put them in Google Drive to share with my sound mixer.  He said there are "dead spots" where the volume graphs are dropping down to zero.  This seems to occur when the tracks overlap and suddenly one or both drop to zero.  This despite the fact that I have fades on all the tracks and never a point where all 3 tracks are at 0, meaning no sound, or where they drop from full sound to nothing.  He sent me a video of what he's seeing -- the volume graphs just drop out/down - no fade - then come back up again.  This is apparently happening throughout the film - I don't know exactly how frequently.  But the point is that that's not what is in my tracks.  Anyone have an idea of what could be happening?  Thank you.