Hello,
I'm kind of puzzled by this. I uploaded some 4K footages and when looking at the preview, they come with horizontal black bars. The thumbnail looks fine but not the preview.
Here the screenshot of the "preview" and the export settings:
Thank you for your feedback!π

I think they try to fit the preview in a 16:9 window. Your footage isn't 16:9 so you get the black bars.

Ohhh, thank you, I see now. All the native 4K have the same black bars.
That helps! Nothing wrong with my files...
Thanks a lot for your answer!