The Gitrog Monster was not triggering on discard because of a null check. As far as I can tell the null check was there just to prevent null-ref but indadvertently caused OncePerEffect triggers to fail triggering when the stack is empty (such as when discarding a land card to pay a cost)

forge-game/src/main/java/forge/game/trigger/TriggerChangesZone.java

@@ -138,7 +138,7 @@ public class TriggerChangesZone extends Trigger {
             // card, rather than keeping track of the host card itself - but it's good enough for now - since there are
             // no cards with multiple different OncePerEffect triggers.
             SpellAbilityStackInstance si = (SpellAbilityStackInstance) runParams2.get("SpellAbilityStackInstance");
-            return si != null && si.attemptOncePerEffectTrigger(this.getHostCard());
+            return si == null || si.attemptOncePerEffectTrigger(this.getHostCard());
         }
 
         return true;