Grey rectangle nested inside of my video, affecting multiple clips in a sequence.

I am making a 4K video consisting of mostly 4K clips, but there are some 1080p clips that are incorporated and scaled up. For some of the clips I have added to the sequence, there is a grey rectangle that shows up on the monitor inside of the clip. This grey rectangle is half the resolution of the 4K clip, so it takes up about the 1080p space inside of the video. It has propagated randomly to multiple video clips in the video. It only affects video clips, not subsequent photos in the video. It does appear to only affect videos clips that occur after, for example, the first 1080p clip occurs in the video. This grey rectangle is still present after I export the video, so it's getting hard coded into the exported file. I have no idea how to remove it, and would appreciate any help you might be able to provide. Thank you!