Before the revamp, thumbnail rows went 4-8-14-20-40, with donation stars needed to unlock the last level; now they go 4-10-20-40.
The old thumbnail cookie values for ranks 2 and 3 were probably tr_4 and tr_7.
I think the uploader is running a sanity check on the cookies which hasn't been updated and doesn't recognize the new tr_5 value, so it bumps you back down to the default.
I just tested with the uconfig value edited to tr_4 and tr_7, and the uploader didn't bump those to tr_2. But tr_8, which would have been another "insane" value, it does bump to tr_2.
Tenboro and Blue Penguin both probably tested with the maximum, tr_20, which it likewise leaves alone because 40 thumbs was supported back when the code was written.
tl;dr try reproducing it with your thumb rows set to 10.