The video from the timeline is dropped in the “sequence” file

B-roll and sound tracks appear perfectly and in sync with the sound in the timeline video. But the timeline video itself drops out to black. I first noticed it after rendering, but when I scrub over the "sequence" file in my assets folder, I see that the drop is occurring there. So the render is properly rendering the sequence. The problem is in the sequence. When I run the video in edit mode, it looks as it should. I am using video shot on my iPhone 13 and editing on  MacBook Pro. I have created several projects without this issue while on the road in Europe (I'm US). This particular project is longer and more complex than my other projects: 10 minutes with ducked sound underneath the timeline video and b-roll over the top of the timeline video. I am having difficulty syncing to creative cloud, perhaps because I am overseas and my enterprise account is hiccuping on that. I have reinstalled Rush. I have ensured all clips are optimized.