Thanks for the detailed log, it made this easy to track down! The bug isn’t in Neo Forge itself. It’s in the script for Garruk, Veiled Butcher in Forge, the engine Neo Forge runs on. His static ability “If a creature an opponent controls would die, exile it instead” is missing a check that the creature is actually on the battlefield. So when his -3 makes your opponent discard creature cards, Garruk “replaces” the discard with an exile that does nothing, and the cards just stay in their hand. Discarding non-creature cards works fine.
Neo Forge is only the interface on top of Forge: the rules and the 30,000+ card scripts are theirs, and I don’t modify their code. That way every Forge update, with its new cards and fixes, reaches Neo Forge without breaking anything. So this one has to be fixed by the Forge team. Once they do, the fix will arrive automatically in the next Neo Forge update.
Until then, keep in mind that while Garruk is on the battlefield, his -3 won’t make an opponent discard creature cards.