--- a/mercurial/branchmap.py	Fri Mar 08 16:47:32 2024 +0100
+++ b/mercurial/branchmap.py	Sat Mar 09 02:07:15 2024 +0100
@@ -157,7 +157,9 @@
             for candidate in (b'base', b'immutable', b'served'):
                 rview = repo.filtered(candidate)
                 if cache.validfor(rview):
-                    cache = self._per_filter[candidate] = cache.copy(rview)
+                    cache._filtername = candidate
+                    self._per_filter[candidate] = cache
+                    cache._dirty = True
                     cache.write(rview)
                     return