diff --git a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlot8GO.cs b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlot8GO.cs
index a1653901b..8b77a4121 100644
--- a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlot8GO.cs
+++ b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlot8GO.cs
@@ -66,6 +66,8 @@ private bool IsMatchPartial(PKM pk)
                 return true;
             if (!GetIVsAboveMinimum(pk))
                 return true;
+            if (!GetIVsBelowMaximum(pk))
+                return true;
 
             // Eevee & Glaceon have different base friendships. Make sure if it is invalid that we yield the other encounter before.
             if (PersonalTable.SWSH.GetFormEntry(Species, Form).BaseFriendship != pk.OT_Friendship)
diff --git a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlotGO.cs b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlotGO.cs
index e37ecbc16..5ba7c8e6a 100644
--- a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlotGO.cs
+++ b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/EncounterSlotGO.cs
@@ -93,15 +93,25 @@ protected override void ApplyDetails(ITrainerInfo sav, EncounterCriteria criteri
                 pk.MetDate = GetRandomValidDate();
             if (Gender != Gender.Random)
                 pk.Gender = (int)Gender;
-            pk.SetRandomIVsGO(Type.GetMinIV());
+            pk.SetRandomIVsGO(Type.GetMinIV(), Type.GetMaxIV());
         }
 
         public bool GetIVsAboveMinimum(PKM pkm)
         {
             int min = Type.GetMinIV();
+            if (min == 0)
+                return false;
             return GetIVsAboveMinimum(pkm, min);
         }
 
+        public bool GetIVsBelowMaximum(PKM pkm)
+        {
+            int max = Type.GetMaxIV();
+            if (max == 15)
+                return false;
+            return GetIVsBelowMaximum(pkm, max);
+        }
+
         private static bool GetIVsAboveMinimum(PKM pkm, int min)
         {
             if (pkm.IV_ATK >> 1 < min) // ATK
@@ -111,10 +121,21 @@ private static bool GetIVsAboveMinimum(PKM pkm, int min)
             return pkm.IV_HP >> 1 >= min; // HP
         }
 
+        private static bool GetIVsBelowMaximum(PKM pkm, int max)
+        {
+            if (pkm.IV_ATK >> 1 > max) // ATK
+                return false;
+            if (pkm.IV_DEF >> 1 > max) // DEF
+                return false;
+            return pkm.IV_HP >> 1 <= max; // HP
+        }
+
         public bool GetIVsValid(PKM pkm)
         {
             if (!GetIVsAboveMinimum(pkm))
                 return false;
+            if (!GetIVsBelowMaximum(pkm))
+                return false;
 
             // HP * 2 | 1 -> HP
             // ATK * 2 | 1 -> ATK&SPA
diff --git a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/PogoType.cs b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/PogoType.cs
index ba0db6762..e1685ee85 100644
--- a/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/PogoType.cs
+++ b/PKHeX.Core/Legality/Encounters/EncounterSlot/GO/PogoType.cs
@@ -31,6 +31,12 @@ public enum PogoType : byte
         GBL,
         /// <summary> GO Battle League Reward (Mythical), requires Lv. 20 and IV = 10 </summary>
         GBLM,
+        /// <summary> GO Battle League Reward, requires Lv. 20 and IV = 0 </summary>
+        /// <remarks> On GO Battle Day (September 18, 2021), IV floor and ceiling were both temporarily set to 0 for non-Legendary encounters. This was fixed at 14:43 UTC (September 17, 2021). </remarks>
+        GBLZero,
+        /// <summary> GO Battle League Reward, requires Lv. 20 and IV = 0 </summary>
+        /// <remarks> On GO Battle Day (September 18, 2021), IV floor was set to 0 after a mishap that also set the IV ceiling to 0. </remarks>
+        GBLDay,
 
         /// <summary> Purified, requires Lv. 8 and IV = 1 (Premier Ball only) </summary>
         Shadow = 30,
@@ -68,9 +74,22 @@ public static class PogoTypeExtensions
             PogoType.ResearchM => 10,
             PogoType.ResearchP => 10,
             PogoType.GBLM => 10,
+            PogoType.GBLZero => 0,
+            PogoType.GBLDay => 0,
             _ => 1,
         };
 
+        /// <summary>
+        /// Gets the minimum IVs (relative to GO's 0-15) the <see cref="encounterType"/> must have.
+        /// </summary>
+        /// <param name="encounterType">Descriptor indicating how the Pokémon was encountered in GO.</param>
+        /// <returns>Required minimum IV (0-15)</returns>
+        public static int GetMaxIV(this PogoType encounterType) => encounterType switch
+        {
+            PogoType.GBLZero => 0,
+            _ => 15,
+        };
+
         /// <summary>
         /// Checks if the <see cref="ball"/> is valid for the <see cref="encounterType"/>.
         /// </summary>
diff --git a/PKHeX.Core/PKM/PKM.cs b/PKHeX.Core/PKM/PKM.cs
index 45bb98784..201b88eae 100644
--- a/PKHeX.Core/PKM/PKM.cs
+++ b/PKHeX.Core/PKM/PKM.cs
@@ -895,13 +895,13 @@ public int[] SetRandomIVs(int? flawless = null)
             return IVs = ivs;
         }
 
-        public int[] SetRandomIVsGO(int minIV = 0)
+        public int[] SetRandomIVsGO(int minIV = 0, int maxIV = 15)
         {
             int[] ivs = new int[6];
             var rnd = Util.Rand;
-            ivs[0] = (rnd.Next(minIV, 16) << 1) | 1; // hp
-            ivs[1] = ivs[4] = (rnd.Next(minIV, 16) << 1) | 1; // attack
-            ivs[2] = ivs[5] = (rnd.Next(minIV, 16) << 1) | 1; // defense
+            ivs[0] = (rnd.Next(minIV, maxIV + 1) << 1) | 1; // hp
+            ivs[1] = ivs[4] = (rnd.Next(minIV, maxIV + 1) << 1) | 1; // attack
+            ivs[2] = ivs[5] = (rnd.Next(minIV, maxIV + 1) << 1) | 1; // defense
             ivs[3] = rnd.Next(MaxIV + 1); // speed
             return IVs = ivs;
         }