--- a/mercurial/localrepo.py	Mon Jan 31 19:28:58 2022 +0100
+++ b/mercurial/localrepo.py	Tue Feb 01 15:19:50 2022 +0100
@@ -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)