diff --git a/src/system/pokemon-data.ts b/src/system/pokemon-data.ts
index 178d2465221..cddc5798872 100644
--- a/src/system/pokemon-data.ts
+++ b/src/system/pokemon-data.ts
@@ -33,7 +33,6 @@ export default class PokemonData {
   public stats: integer[];
   public ivs: integer[];
   public nature: Nature;
-  public natureOverride: Nature | -1;
   public moveset: (PokemonMove | null)[];
   public status: Status | null;
   public friendship: integer;
@@ -54,14 +53,20 @@ export default class PokemonData {
   public fusionVariant: Variant;
   public fusionGender: Gender;
   public fusionLuck: integer;
-  public fusionCustomPokemonData: CustomPokemonData;
 
   public boss: boolean;
   public bossSegments?: integer;
 
   public summonData: PokemonSummonData;
+
   /** Data that can customize a Pokemon in non-standard ways from its Species */
   public customPokemonData: CustomPokemonData;
+  public fusionCustomPokemonData: CustomPokemonData;
+
+  // Deprecated attributes, needed for now to allow SessionData migration (see PR#4619 comments)
+  public natureOverride: Nature | -1;
+  public mysteryEncounterPokemonData: CustomPokemonData | null;
+  public fusionMysteryEncounterPokemonData: CustomPokemonData | null;
 
   constructor(source: Pokemon | any, forHistory: boolean = false) {
     const sourcePokemon = source instanceof Pokemon ? source : null;
@@ -112,6 +117,9 @@ export default class PokemonData {
 
     this.customPokemonData = new CustomPokemonData(source.customPokemonData);
 
+    this.mysteryEncounterPokemonData = new CustomPokemonData(source.mysteryEncounterPokemonData);
+    this.fusionMysteryEncounterPokemonData = new CustomPokemonData(source.fusionMysteryEncounterPokemonData);
+
     if (!forHistory) {
       this.boss = (source instanceof EnemyPokemon && !!source.bossSegments) || (!this.player && !!source.boss);
       this.bossSegments = source.bossSegments;
diff --git a/src/system/version_migration/versions/v1_1_0.ts b/src/system/version_migration/versions/v1_1_0.ts
index b60822a60f6..aac554c4531 100644
--- a/src/system/version_migration/versions/v1_1_0.ts
+++ b/src/system/version_migration/versions/v1_1_0.ts
@@ -14,15 +14,18 @@ export const sessionMigrators = [
   function migrateCustomPokemonDataAndNatureOverrides(data: SessionSaveData) {
     // Fix Pokemon nature overrides and custom data migration
     data.party.forEach(pokemon => {
-      if (pokemon["mysteryEncounterData"]) {
-        pokemon.customPokemonData = new CustomPokemonData(pokemon["mysteryEncounterData"]);
+      if (pokemon["mysteryEncounterPokemonData"]) {
+        pokemon.customPokemonData = new CustomPokemonData(pokemon["mysteryEncounterPokemonData"]);
+        pokemon["mysteryEncounterPokemonData"] = null;
       }
-      if (pokemon["fusionMysteryEncounterData"]) {
-        pokemon.fusionCustomPokemonData = new CustomPokemonData(pokemon["fusionMysteryEncounterData"]);
+      if (pokemon["fusionMysteryEncounterPokemonData"]) {
+        pokemon.fusionCustomPokemonData = new CustomPokemonData(pokemon["fusionMysteryEncounterPokemonData"]);
+        pokemon["fusionMysteryEncounterPokemonData"] = null;
       }
       pokemon.customPokemonData = pokemon.customPokemonData ?? new CustomPokemonData();
-      if (pokemon["natureOverride"]) {
+      if (pokemon["natureOverride"] && pokemon["natureOverride"] >= 0) {
         pokemon.customPokemonData.nature = pokemon["natureOverride"];
+        pokemon["natureOverride"] = -1;
       }
     });
   }