diff --git a/src/AcDream.App/Rendering/CellVisibility.cs b/src/AcDream.App/Rendering/CellVisibility.cs
index d3cf6f1..87fdc67 100644
--- a/src/AcDream.App/Rendering/CellVisibility.cs
+++ b/src/AcDream.App/Rendering/CellVisibility.cs
@@ -176,6 +176,18 @@ public sealed class CellVisibility
     /// <summary>Full-ID lookup for O(1) neighbour resolution during BFS.</summary>
     private readonly Dictionary<uint, LoadedCell> _cellLookup = new();
 
+    /// <summary>
+    /// Phase A8 RR7.1 (2026-05-27): read-only view of every loaded cell, keyed
+    /// by full 32-bit cell id. Used by <see cref="Wb.BuildingLoader"/> at
+    /// landblock-info-arrival time so its BFS can reach cells that streamed
+    /// in on earlier frames (not just the per-frame drain). Without this view
+    /// the registry's <c>EnvCellIds</c> set was systematically short, leaving
+    /// <see cref="LoadedCell.BuildingId"/> unset on the cells the camera
+    /// actually enters — which silently routed indoor frames through the
+    /// outdoor branch.
+    /// </summary>
+    public IReadOnlyDictionary<uint, LoadedCell> AllLoadedCells => _cellLookup;
+
     /// <summary>The cell the camera was in during the last <see cref="ComputeVisibility"/> call.</summary>
     private LoadedCell? _lastCameraCell;
 
diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index d6ecebb..6c26170 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -5696,12 +5696,21 @@ public sealed class GameWindow : IDisposable
         _terrain.AddLandblockWithMesh(lb.LandblockId, meshData, origin);
 
         // Step 4: drain pending LoadedCells from the worker thread.
-        // Also collect into a local dict for the BuildingLoader stamping pass below.
-        var drainedCells = new System.Collections.Generic.Dictionary<uint, LoadedCell>();
+        // Phase A8 RR7.1 (2026-05-27): also late-stamp BuildingId on each
+        // arriving cell if the landblock's BuildingRegistry already exists
+        // (cells loaded after the registry-build pass at line ~5876). Cells
+        // arriving BEFORE the registry are stamped by BuildingLoader.Build
+        // itself via the AllLoadedCells dict.
         while (_pendingCells.TryTake(out var cell))
         {
             _cellVisibility.AddCell(cell);
-            drainedCells[cell.CellId] = cell;
+            uint cellLbId = cell.CellId & 0xFFFF0000u;
+            if (_buildingRegistries.TryGetValue(cellLbId, out var existingReg))
+            {
+                var bs = existingReg.GetBuildingsContainingCell(cell.CellId);
+                if (bs.Count > 0)
+                    cell.BuildingId = bs[0].BuildingId;
+            }
         }
 
         // Compute the per-landblock AABB for frustum culling. XY from the
@@ -5863,18 +5872,20 @@ public sealed class GameWindow : IDisposable
             _physicsEngine.AddLandblock(lb.LandblockId, terrainSurface, cellSurfaces,
                 portalPlanes, origin.X, origin.Y);
 
-            // Phase A8 (2026-05-26): build per-landblock BuildingRegistry from
-            // LandBlockInfo.Buildings, stamping LoadedCell.BuildingId for each cell
-            // in a building's cell set. Uses the already-drained drainedCells dict
-            // (LoadedCells registered this frame) so stamping and registry build
-            // happen in the same render-thread pass — no extra dat reads required.
-            // Cells without a building stay at BuildingId == null (outdoor surface
-            // cells; dungeon cells not enumerated in LandBlockInfo.Buildings).
+            // Phase A8 (2026-05-26, fixed 2026-05-27 RR7.1): build per-landblock
+            // BuildingRegistry from LandBlockInfo.Buildings, stamping
+            // LoadedCell.BuildingId for each cell in a building's cell set.
+            // Uses _cellVisibility.AllLoadedCells (every cell loaded so far,
+            // not just the per-frame drain) so the BFS can reach cells that
+            // streamed in on earlier frames. Cells arriving AFTER this build
+            // pass get stamped at drain time (see _pendingCells loop above).
+            // Cells without a building stay at BuildingId == null (outdoor
+            // surface cells; dungeon cells not in LandBlockInfo.Buildings).
             if (lbInfo is not null)
             {
                 _buildingRegistries[lb.LandblockId] =
                     AcDream.App.Rendering.Wb.BuildingLoader.Build(
-                        lbInfo, lb.LandblockId, drainedCells);
+                        lbInfo, lb.LandblockId, _cellVisibility.AllLoadedCells);
             }
         }