There is a discrepancy between how Movie Scene Capture (Legacy) and Movie Render Queue handle letterboxing for sensor aspect ratio's that do not match the export resolution, which seems to be a bug with Movie Render Queue specifically for sensor aspect ratio's between 16:9 (1.78) and 1:1 (1), demonstrated in the attached GIF:
The frames have letterboxes burned in on render from either render method for most of the range:
The exception is those frames with a sensor aspect ratio between 16:9 and 1:1 do not have their letterbox burned in on the MRQ render only:
Using [Link Removed] (4.27):
Or from scratch in 4.26 or 4.27 launcher versions:
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-120483 in the post.