diff --git a/OpenRA.Game/Map/Map.cs b/OpenRA.Game/Map/Map.cs
index 333aeaffa9..9921589829 100644
--- a/OpenRA.Game/Map/Map.cs
+++ b/OpenRA.Game/Map/Map.cs
@@ -243,6 +243,8 @@ namespace OpenRA
 		public CellRegion AllCells { get; private set; }
 		public List<CPos> AllEdgeCells { get; private set; }
 
+		public event Action<CPos> CellProjectionChanged;
+
 		// Internal data
 		readonly ModData modData;
 		CellLayer<short> cachedTerrainIndexes;
@@ -336,11 +338,12 @@ namespace OpenRA
 			Height = new CellLayer<byte>(Grid.Type, size);
 			Ramp = new CellLayer<byte>(Grid.Type, size);
 			Tiles.Clear(terrainInfo.DefaultTerrainTile);
+
 			if (Grid.MaximumTerrainHeight > 0)
 			{
-				Height.CellEntryChanged += UpdateProjection;
-				Tiles.CellEntryChanged += UpdateProjection;
 				Tiles.CellEntryChanged += UpdateRamp;
+				Tiles.CellEntryChanged += UpdateProjection;
+				Height.CellEntryChanged += UpdateProjection;
 			}
 
 			PostInit();
@@ -530,6 +533,7 @@ namespace OpenRA
 				var inverse = inverseCellProjection[uv];
 				inverse.Clear();
 				inverse.Add(uv);
+				CellProjectionChanged?.Invoke(cell);
 				return;
 			}
 
@@ -567,6 +571,8 @@ namespace OpenRA
 					projectedHeight[temp] = height;
 				}
 			}
+
+			CellProjectionChanged?.Invoke(cell);
 		}
 
 		byte ProjectedCellHeightInner(PPos puv)
diff --git a/OpenRA.Mods.Common/Pathfinder/HierarchicalPathFinder.cs b/OpenRA.Mods.Common/Pathfinder/HierarchicalPathFinder.cs
index 7ba38b5692..885faed296 100644
--- a/OpenRA.Mods.Common/Pathfinder/HierarchicalPathFinder.cs
+++ b/OpenRA.Mods.Common/Pathfinder/HierarchicalPathFinder.cs
@@ -281,6 +281,10 @@ namespace OpenRA.Mods.Common.Pathfinder
 			// When we build the cost table, it depends on the movement costs of the cells at that time.
 			// When this changes, we must update the cost table.
 			locomotor.CellCostChanged += RequireCostRefreshInCell;
+
+			// If the map projection changes, the result of Map.Contains(CPos) may change.
+			// We need to rebuild grids to account for this possibility.
+			this.world.Map.CellProjectionChanged += RequireProjectionRefreshInCell;
 		}
 
 		public (
@@ -619,6 +623,14 @@ namespace OpenRA.Mods.Common.Pathfinder
 			}
 		}
 
+		/// <summary>
+		/// When map projection changes for a cell, marks the grid it belongs to as out of date.
+		/// </summary>
+		void RequireProjectionRefreshInCell(CPos cell)
+		{
+			dirtyGridIndexes.Add(GridIndex(cell));
+		}
+
 		/// <summary>
 		/// <see cref="BlockedByActor.Immovable"/> defines immovability based on the mobile trait. The blocking rules
 		/// in <see cref="Locomotor.CanMoveFreelyInto(Actor, CPos, SubCell, BlockedByActor, Actor)"/> allow units to