Odd... but the timeline previews are of little consequence
if your exports are correct.
Have you exported successfully without error?
Yes, exporting seems to be fine. It's not a dealbreaker but it's an annoyance that I'd like to fix nonetheless.
Is this a known problem (with maybe even a known solution)?
I have never seen this specifically reported as a problem,
but I would suspect the anamorphic pixels in your 2048x858.
They're not anamorphic. It's completely normal square pixels.
It does seem to have something to do with the sequence's aspect ratio though because it works with other aspect ratios.
Perhaps Premiere is incorrectly interpreting
the par as anamorphic for preview renders?
Again... this particular issue is alien to me.