diff --git a/google/cloud/pubsublite/cloudpubsub/internal/ack_set_tracker_impl.py b/google/cloud/pubsublite/cloudpubsub/internal/ack_set_tracker_impl.py
index 3b10d3c1..5a648091 100644
--- a/google/cloud/pubsublite/cloudpubsub/internal/ack_set_tracker_impl.py
+++ b/google/cloud/pubsublite/cloudpubsub/internal/ack_set_tracker_impl.py
@@ -54,7 +54,7 @@ async def ack(self, offset: int):
             if receipt == ack:
                 prefix_acked_offset = receipt
                 continue
-            self._receipts.append(receipt)
+            self._receipts.appendleft(receipt)
             self._acks.put(ack)
             break
         if prefix_acked_offset is None:
diff --git a/tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py b/tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py
index 52a5d317..8824d89e 100644
--- a/tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py
+++ b/tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py
@@ -51,15 +51,17 @@ async def test_track_and_aggregate_acks(committer, tracker: AckSetTracker):
         committer.commit.assert_has_calls([])
         await tracker.ack(offset=3)
         committer.commit.assert_has_calls([])
-        await tracker.ack(offset=5)
-        committer.commit.assert_has_calls([])
         await tracker.ack(offset=1)
-        committer.commit.assert_has_calls([call(Cursor(offset=6))])
+        committer.commit.assert_has_calls([call(Cursor(offset=4))])
+        await tracker.ack(offset=5)
+        committer.commit.assert_has_calls(
+            [call(Cursor(offset=4)), call(Cursor(offset=6))]
+        )
 
         tracker.track(offset=8)
         await tracker.ack(offset=7)
         committer.commit.assert_has_calls(
-            [call(Cursor(offset=6)), call(Cursor(offset=8))]
+            [call(Cursor(offset=4)), call(Cursor(offset=6)), call(Cursor(offset=8))]
         )
     committer.__aexit__.assert_called_once()