Btw - since you now have that ISS flythrough video - that was the one causing issues when exporting to Prores within a 2k project. Meanwhile, though, I did more tests, and it doesn't seem to be a general downscaling issue with exports, I was able to create both immediate 4k exports from lower res content, as well as 2k exports from full 4k res content, both from and into mp4 and Prores, also from 4k DCPs. Maybe that ISS video is just a bit special.
I guess it's rather exotic to export non j2k source content immediately anyway, although, for subtitle screeners etc. some people may go that way, and then probably in 2k as well. Not a show stopper, as mp4 works, and the higher quality Prores export is probably mostly chosen for j2c content.