diff --git a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
index c7febd917..ca0733f40 100644
--- a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
+++ b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
@@ -3941,7 +3941,13 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
         CGFloat viewTopToContentBottom = self.safeContentHeight - self.collectionView.contentOffset.y;
 
         NSUInteger oldCellCount = [self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
+
         [self resetMappings];
+
+        // ViewItem's modified while we were not observing may be stale.
+        // TODO have a more fine-grained cache expiration based on rows modified.
+        [self.viewItemMap removeAllObjects];
+
         NSUInteger newCellCount = [self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
 
         // Detect changes in the mapping's "window" size.