diff --git a/gateway/session.py b/gateway/session.py
index 9fa3ed15c7..b0a6c798f4 100644
--- a/gateway/session.py
+++ b/gateway/session.py
@@ -1425,7 +1425,14 @@ class SessionStore:
                 "Could not resolve profile home for %r: %s", session_key, exc
             )
             home = None
-        cache[profile] = home
+        # Only a hit is memoized. A profile directory can appear *after* the
+        # gateway started — the enrollment bridge provisions profiles/<name>/
+        # at runtime — and caching the miss would pin that profile's rows to
+        # the ambient store for the life of the process, which is the bug
+        # this helper exists to prevent. A miss costs one profile_exists()
+        # stat and only recurs for profiles that genuinely do not exist.
+        if home is not None:
+            cache[profile] = home
         return home
 
     def _db_for_key(self, session_key: Optional[str]):
diff --git a/tests/gateway/test_multiplex_session_db_profile_scope.py b/tests/gateway/test_multiplex_session_db_profile_scope.py
index ca6580ef5f..5980248519 100644
--- a/tests/gateway/test_multiplex_session_db_profile_scope.py
+++ b/tests/gateway/test_multiplex_session_db_profile_scope.py
@@ -606,3 +606,22 @@ def test_pinned_handle_still_wins_over_key_resolution(multiplex_homes):
     store._db = sentinel
     assert store._db_for_key("agent:fitness:telegram:dm:1") is sentinel
     assert store._db_for_session_id("whatever") is sentinel
+
+
+def test_profile_home_is_not_memoized_before_the_profile_exists(multiplex_homes):
+    """A profile provisioned after startup must not stay pinned to the root store.
+
+    The enrollment bridge creates ``profiles/<name>/`` at runtime, so a key can
+    be seen before its directory exists.  Memoizing that miss would pin the
+    profile to the ambient store for the life of the process — the exact bug
+    this helper exists to prevent.
+    """
+    root, _profile = multiplex_homes
+    store = _multiplex_store(root)
+    key = "agent:latecomer:telegram:dm:9"
+
+    assert store._profile_home_for_key(key) is None
+
+    (root / "profiles" / "latecomer").mkdir(parents=True)
+
+    assert store._profile_home_for_key(key) == root / "profiles" / "latecomer"