There’s an issue with the umon 1-25 columns in levels.txt file. The column is supposed to dictate what kind of enemies can become champions and uniques, but this column works only for normal difficulty.
If it worked for nightmare and hell as well, it would allow modders implement special encounters in endgame zones without having to worry about those monster types becoming champions and uniques. It would open a ton of possibilities for mods. Please consider changing it to work on nightmare and hell as well.