diff --git a/changelog.d/18103.bugfix b/changelog.d/18103.bugfix
new file mode 100644
index 00000000000..11000d569fd
--- /dev/null
+++ b/changelog.d/18103.bugfix
@@ -0,0 +1,3 @@
+Fixed a bug that disturbed room creation when a check_event_allowed callback was registered by a module and power_level Notifications mapping exists.
+
+Contributed by @c-cal
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 8e9d27138ca..85d1f63bbc7 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -47,7 +47,7 @@
 from synapse.synapse_rust.events import EventInternalMetadata
 from synapse.types import JsonDict, StrCollection
 from synapse.util.caches import intern_dict
-from synapse.util.frozenutils import freeze
+from synapse.util.frozenutils import freeze, unfreeze
 from synapse.util.stringutils import strtobool
 
 if TYPE_CHECKING:
@@ -319,6 +319,12 @@ def freeze(self) -> None:
         # this will be a no-op if the event dict is already frozen.
         self._dict = freeze(self._dict)
 
+    def unfreeze(self) -> None:
+        """'Unfreeze' the event dict, so it can be modified afterwards"""
+
+        # this will be a no-op if the event dict is already unfrozen.
+        self._dict = unfreeze(self._dict)
+
     def __str__(self) -> str:
         return self.__repr__()
 
diff --git a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
index 9f7a04372de..6672f780ccc 100644
--- a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
+++ b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
@@ -293,6 +293,9 @@ async def check_event_allowed(
         # the hashes and signatures.
         event.freeze()
 
+        allow = True
+        event_dict = None
+
         for callback in self._check_event_allowed_callbacks:
             try:
                 res, replacement_data = await delay_cancellation(
@@ -318,11 +321,15 @@ async def check_event_allowed(
             # Return if the event shouldn't be allowed or if the module came up with a
             # replacement dict for the event.
             if res is False:
-                return res, None
+                allow = False
+                break
             elif isinstance(replacement_data, dict):
-                return True, replacement_data
+                event_dict = replacement_data
+                break
+
+        event.unfreeze()
 
-        return True, None
+        return allow, event_dict
 
     async def on_create_room(
         self, requester: Requester, config: dict, is_requester_admin: bool