diff --git a/Content.Server/Atmos/EntitySystems/AtmosphereSystem.Monstermos.cs b/Content.Server/Atmos/EntitySystems/AtmosphereSystem.Monstermos.cs
index cca529fd58..24463442cd 100644
--- a/Content.Server/Atmos/EntitySystems/AtmosphereSystem.Monstermos.cs
+++ b/Content.Server/Atmos/EntitySystems/AtmosphereSystem.Monstermos.cs
@@ -596,8 +596,17 @@ namespace Content.Server.Atmos.EntitySystems
             if (!reconsiderAdjacent)
                 return;
 
+            // Before updating the adjacent tile flags that determine whether air is allowed to flow
+            // or not, we explicitly update airtight data on these tiles right now.
+            // This ensures that UpdateAdjacentTiles has updated data before updating flags.
+            // This allows monstermos' floodfill check that determines if firelocks have dropped
+            // to work correctly.
+            UpdateAirtightData(ent.Owner, ent.Comp1, ent.Comp3, tile);
+            UpdateAirtightData(ent.Owner, ent.Comp1, ent.Comp3, other);
+
             UpdateAdjacentTiles(ent, tile);
             UpdateAdjacentTiles(ent, other);
+
             InvalidateVisuals(ent, tile);
             InvalidateVisuals(ent, other);
         }