It appears that version 1.9.9-backport.1 (MC 1.19.2) is no longer respecting the difficulty blacklist in the config file. I set several mobs to spawn only on Expert and Master mode and they are appearing at the beginning of the game.

In previous versions this was not happening.

Other information that may be useful
Forge version: 43.3.8
I'm using this mod in a huge modpack

Apparently I was the one who did something wrong. I went to change the configuration file today and realized that I had left an error. I fixed it and it seems that things are working again as I configured them.