I have the same problem with my UR22 mkII, it’s just slightly noticable but it drives me crazy. Just to be sure, I connect the output from my UAD Apollo Twin to the input in my UR22, and when i bumped the input volume on the left channel, it sounded way better.
I tried sending it back to Thomann and they sent me a new one (at least it looked new), but it’s still the same problem, so now I don’t even know what the problem could be anymore.
Just wanted to follow this thread in case somebody figures it out.