diff --git a/changelog.d/15002.misc b/changelog.d/15002.misc
new file mode 100644
index 000000000..68ac8335f
--- /dev/null
+++ b/changelog.d/15002.misc
@@ -0,0 +1 @@
+Add tests for `_flatten_dict`.
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index f73dceb12..d9c0a98f4 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -36,7 +36,7 @@ from synapse.api.constants import (
     Membership,
     RelationTypes,
 )
-from synapse.api.room_versions import PushRuleRoomFlag, RoomVersion
+from synapse.api.room_versions import PushRuleRoomFlag
 from synapse.event_auth import auth_types_for_event, get_user_power_level
 from synapse.events import EventBase, relation_from_event
 from synapse.events.snapshot import EventContext
@@ -405,7 +405,7 @@ class BulkPushRuleEvaluator:
             room_mention = mentions.get("room") is True
 
         evaluator = PushRuleEvaluator(
-            _flatten_dict(event, room_version=event.room_version),
+            _flatten_dict(event),
             has_mentions,
             user_mentions,
             room_mention,
@@ -491,7 +491,6 @@ StateGroup = Union[object, int]
 
 def _flatten_dict(
     d: Union[EventBase, Mapping[str, Any]],
-    room_version: Optional[RoomVersion] = None,
     prefix: Optional[List[str]] = None,
     result: Optional[Dict[str, str]] = None,
 ) -> Dict[str, str]:
@@ -511,7 +510,6 @@ def _flatten_dict(
 
     Args:
         d: The event or content to continue flattening.
-        room_version: The room version object.
         prefix: The key prefix (from outer dictionaries).
         result: The result to mutate.
 
@@ -531,14 +529,13 @@ def _flatten_dict(
 
     # `room_version` should only ever be set when looking at the top level of an event
     if (
-        room_version is not None
-        and PushRuleRoomFlag.EXTENSIBLE_EVENTS in room_version.msc3931_push_features
-        and isinstance(d, EventBase)
+        isinstance(d, EventBase)
+        and PushRuleRoomFlag.EXTENSIBLE_EVENTS in d.room_version.msc3931_push_features
     ):
         # Room supports extensible events: replace `content.body` with the plain text
         # representation from `m.markup`, as per MSC1767.
         markup = d.get("content").get("m.markup")
-        if room_version.identifier.startswith("org.matrix.msc1767."):
+        if d.room_version.identifier.startswith("org.matrix.msc1767."):
             markup = d.get("content").get("org.matrix.msc1767.markup")
         if markup is not None and isinstance(markup, list):
             text = ""
diff --git a/tests/push/test_push_rule_evaluator.py b/tests/push/test_push_rule_evaluator.py
index 7c430c4ec..da3342387 100644
--- a/tests/push/test_push_rule_evaluator.py
+++ b/tests/push/test_push_rule_evaluator.py
@@ -22,7 +22,7 @@ import synapse.rest.admin
 from synapse.api.constants import EventTypes, HistoryVisibility, Membership
 from synapse.api.room_versions import RoomVersions
 from synapse.appservice import ApplicationService
-from synapse.events import FrozenEvent
+from synapse.events import FrozenEvent, make_event_from_dict
 from synapse.push.bulk_push_rule_evaluator import _flatten_dict
 from synapse.push.httppusher import tweaks_for_actions
 from synapse.rest import admin
@@ -60,6 +60,67 @@ class FlattenDictTestCase(unittest.TestCase):
         }
         self.assertEqual({"woo": "woo"}, _flatten_dict(input))
 
+    def test_event(self) -> None:
+        """Events can also be flattened."""
+        event = make_event_from_dict(
+            {
+                "room_id": "!test:test",
+                "type": "m.room.message",
+                "sender": "@alice:test",
+                "content": {
+                    "msgtype": "m.text",
+                    "body": "Hello world!",
+                    "format": "org.matrix.custom.html",
+                    "formatted_body": "<h1>Hello world!</h1>",
+                },
+            },
+            room_version=RoomVersions.V8,
+        )
+        expected = {
+            "content.msgtype": "m.text",
+            "content.body": "hello world!",
+            "content.format": "org.matrix.custom.html",
+            "content.formatted_body": "<h1>hello world!</h1>",
+            "room_id": "!test:test",
+            "sender": "@alice:test",
+            "type": "m.room.message",
+        }
+        self.assertEqual(expected, _flatten_dict(event))
+
+    def test_extensible_events(self) -> None:
+        """Extensible events has compatibility behaviour."""
+        event_dict = {
+            "room_id": "!test:test",
+            "type": "m.room.message",
+            "sender": "@alice:test",
+            "content": {
+                "org.matrix.msc1767.markup": [
+                    {"mimetype": "text/plain", "body": "Hello world!"},
+                    {"mimetype": "text/html", "body": "<h1>Hello world!</h1>"},
+                ]
+            },
+        }
+
+        # For a current room version, there's no special behavior.
+        event = make_event_from_dict(event_dict, room_version=RoomVersions.V8)
+        expected = {
+            "room_id": "!test:test",
+            "sender": "@alice:test",
+            "type": "m.room.message",
+        }
+        self.assertEqual(expected, _flatten_dict(event))
+
+        # For a room version with extensible events, they parse out the text/plain
+        # to a content.body property.
+        event = make_event_from_dict(event_dict, room_version=RoomVersions.MSC1767v10)
+        expected = {
+            "content.body": "hello world!",
+            "room_id": "!test:test",
+            "sender": "@alice:test",
+            "type": "m.room.message",
+        }
+        self.assertEqual(expected, _flatten_dict(event))
+
 
 class PushRuleEvaluatorTestCase(unittest.TestCase):
     def _get_evaluator(