I was trying to run JER 1.16 with Pixelmon but something about the way that Pixelmon named entities was causing MobHelper.getExpandedName to receive empty strings. Since it doesn't check that strings are of at least length 1 before trying to get a substring, these entity names cause a crash that prevents all mob loot from loading in, with the end result being that JER doesn't seem to appear in-game at all.