diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -149,6 +149,13 @@
     def clear(self):
         self._per_filter.clear()
 
+    def write_delayed(self, repo):
+        unfi = repo.unfiltered()
+        for filtername, cache in self._per_filter.items():
+            if cache._delayed:
+                repo = unfi.filtered(filtername)
+                cache.write(repo)
+
 
 def _unknownnode(node):
     """raises ValueError when branchcache found a node which does not exists"""
@@ -199,6 +206,7 @@
         has a given node or not. If it's not provided, we assume that every node
         we have exists in changelog"""
         self._repo = repo
+        self._delayed = False
         if tipnode is None:
             self.tipnode = repo.nullid
         else:
@@ -403,6 +411,13 @@
         )
 
     def write(self, repo):
+        tr = repo.currenttransaction()
+        if not getattr(tr, 'finalized', True):
+            # Avoid premature writing.
+            #
+            # (The cache warming setup by localrepo will update the file later.)
+            self._delayed = True
+            return
         try:
             f = repo.cachevfs(self._filename(repo), b"w", atomictemp=True)
             cachekey = [hex(self.tipnode), b'%d' % self.tiprev]
@@ -427,6 +442,7 @@
                 len(self._entries),
                 nodecount,
             )
+            self._delayed = False
         except (IOError, OSError, error.Abort) as inst:
             # Abort may be raised by read only opener, so log and continue
             repo.ui.debug(
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -2830,6 +2830,8 @@
                 self.ui.debug(b'updating the branch cache\n')
                 self.filtered(b'served').branchmap()
                 self.filtered(b'served.hidden').branchmap()
+                # flush all possibly delayed write.
+                self._branchcaches.write_delayed(self)
 
         if repository.CACHE_CHANGELOG_CACHE in caches:
             self.changelog.update_caches(transaction=tr)
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -1336,7 +1336,5 @@
   abort: pretxnclose hook failed
   [40]
   $ cat branchmap-update-02/.hg/cache/branch2-served
-  99ba08759bc7f6fdbe5304e83d0387f35c082479 1 (missing-correct-output !)
-  99ba08759bc7f6fdbe5304e83d0387f35c082479 o A (missing-correct-output !)
-  71ca9a6d524ed3c2a215119b2086ac3b8c4c8286 3 (known-bad-output !)
-  71ca9a6d524ed3c2a215119b2086ac3b8c4c8286 o A (known-bad-output !)
+  99ba08759bc7f6fdbe5304e83d0387f35c082479 1
+  99ba08759bc7f6fdbe5304e83d0387f35c082479 o A