diff --git a/cull/Services/PhotoImporter.swift b/cull/Services/PhotoImporter.swift
--- a/cull/Services/PhotoImporter.swift
+++ b/cull/Services/PhotoImporter.swift
@@ -97,16 +97,18 @@ collected.append(result)
                 }
                 return collected
             }
-            // Apply on main actor
-            for (index, meta) in results {
-                let photo = photos[index]
-                photo.captureDate = meta.captureDate
-                photo.pixelWidth = meta.pixelWidth
-                photo.pixelHeight = meta.pixelHeight
-                photo.fileSize = meta.fileSize
-                photo.pairedPixelWidth = meta.pairedPixelWidth
-                photo.pairedPixelHeight = meta.pairedPixelHeight
-                photo.pairedFileSize = meta.pairedFileSize
+            // Apply metadata on main actor (@Observable Photo requires it)
+            await MainActor.run {
+                for (index, meta) in results {
+                    let photo = photos[index]
+                    photo.captureDate = meta.captureDate
+                    photo.pixelWidth = meta.pixelWidth
+                    photo.pixelHeight = meta.pixelHeight
+                    photo.fileSize = meta.fileSize
+                    photo.pairedPixelWidth = meta.pairedPixelWidth
+                    photo.pairedPixelHeight = meta.pairedPixelHeight
+                    photo.pairedFileSize = meta.pairedFileSize
+                }
             }
         }
 
diff --git a/cull/Services/ShotGrouper.swift b/cull/Services/ShotGrouper.swift
--- a/cull/Services/ShotGrouper.swift
+++ b/cull/Services/ShotGrouper.swift
@@ -147,8 +147,9 @@ let shouldMerge = areGroupsCloseInTime(previous, current) &&
                               areGroupsVisuallySimilar(previous, current, featurePrintMap: featurePrintMap)
 
             if shouldMerge {
-                // Merge into previous
-                previous.photos.append(contentsOf: current.photos)
+                // Build a new group with combined photos instead of mutating the existing one
+                let combined = PhotoGroup(photos: previous.photos + current.photos)
+                merged[merged.count - 1] = combined
             } else {
                 merged.append(current)
             }
diff --git a/cull/Services/ThumbnailCache.swift b/cull/Services/ThumbnailCache.swift
--- a/cull/Services/ThumbnailCache.swift
+++ b/cull/Services/ThumbnailCache.swift
@@ -155,6 +155,7 @@
         Task.detached(priority: .utility) {
             for batchStart in stride(from: 0, to: work.count, by: 8) {
                 let batch = Array(work[batchStart..<min(batchStart + 8, work.count)])
+                var batchKeys: [String] = []
                 await withTaskGroup(of: (String, NSImage?).self) { group in
                     for (key, loadURL, cacheKeyURL) in batch {
                         let diskPath = diskCache.appendingPathComponent(Self.stableDiskKey(for: cacheKeyURL))
@@ -173,7 +174,16 @@ for await (key, image) in group {
                         if let image {
                             // NSCache is thread-safe, no need for MainActor
                             mc.setObject(image, forKey: key as NSString)
+                            batchKeys.append(key)
                         }
+                    }
+                }
+                // Key tracking and generation bump must happen on MainActor
+                if !batchKeys.isEmpty {
+                    await MainActor.run { [weak self] in
+                        guard let self else { return }
+                        for k in batchKeys { self.thumbnailKeys.insert(k) }
+                        self.cacheGeneration += 1
                     }
                 }
             }