Firstly, the Selective Mob Griefing feature is a great addition and could be very useful being part of this mod over having to try finding a mod just for this purpose, and that addition is what lead me to encounter this issue:

Game Nerfs does not save as being enabled in-game (I enable it and save, but then immediately to re-check, it's instead disabled, and this is the case for every attempt). Despite that the config file always shows it set to "true", that doesn't seem to matter -- since it doesn't save in-game, Game Nerfs and its sub-categories won't activate. To note, the Selective Mob Griefing settings (in this case, being enabled and with few other mobs added to the list) does save, given it's always displaying as enabled every time I check. It's just Game Nerfs that resets itself to being disabled, so that seems to be a bug.

Can you post a copy of the config and the game log?

I had figured a workaround by adding a separate mod for mob-griefing specifics with Quark's "Game Nerfs" disabled, which all works out. Being part of a custom modpack, some mod must have conflicts to somehow force-disable "Game Nerfs" (since if Quark is by itself, it retains being enabled as expected).