[Bug] Moved code preventing MBH's transfer to post modifier generation

src/game-mode.ts

@@ -230,6 +230,13 @@ export class GameMode implements GameModeConfig {
     return waveIndex % 10 === 0;
   }
 
+  /**
+   * @returns `true` if the current battle is against classic mode's final boss
+   */
+  isBattleClassicFinalBoss(waveIndex: number): boolean {
+    return (this.modeId === GameModes.CLASSIC || this.modeId === GameModes.CHALLENGE) && this.isWaveFinal(waveIndex);
+  }
+
   /**
      * Every 50 waves of an Endless mode is a boss
      * At this time it is paradox pokemon

src/phases/encounter-phase.ts

@@ -141,10 +141,6 @@ export class EncounterPhase extends BattlePhase {
         } else if (!(battle.waveIndex % 1000)) {
           enemyPokemon.formIndex = 1;
           enemyPokemon.updateScale();
-          const bossMBH = this.scene.findModifier(m => m instanceof TurnHeldItemTransferModifier && m.pokemonId === enemyPokemon.id, false) as TurnHeldItemTransferModifier;
-          this.scene.removeModifier(bossMBH!);
-          bossMBH?.setTransferrableFalse();
-          this.scene.addEnemyModifier(bossMBH!);
         }
       }
 
@@ -442,6 +438,15 @@ export class EncounterPhase extends BattlePhase {
       if (enemyPokemon.isShiny()) {
         this.scene.unshiftPhase(new ShinySparklePhase(this.scene, BattlerIndex.ENEMY + e));
       }
+      /** This sets Eternatus' held item to be untransferrable, preventing it from being stolen  */
+      if (enemyPokemon.species.speciesId === Species.ETERNATUS && (this.scene.gameMode.isBattleClassicFinalBoss(this.scene.currentBattle.waveIndex) || this.scene.gameMode.isEndlessMajorBoss(this.scene.currentBattle.waveIndex))) {
+        const enemyMBH = this.scene.findModifier(m => m instanceof TurnHeldItemTransferModifier, false) as TurnHeldItemTransferModifier;
+        if (enemyMBH) {
+          this.scene.removeModifier(enemyMBH, true);
+          enemyMBH.setTransferrableFalse();
+          this.scene.addEnemyModifier(enemyMBH);
+        }
+      }
     });
 
     if (![ BattleType.TRAINER, BattleType.MYSTERY_ENCOUNTER ].includes(this.scene.currentBattle.battleType)) {