Turns out g_s_e_id is always in the payload, just set as None

hikari/impl/entity_factory.py

@@ -1476,7 +1476,7 @@ def deserialize_stage_instance(self, payload: data_binding.JSONObject) -> stage_
             topic=payload["topic"],
             discoverable_disabled=payload["discoverable_disabled"],
             guild_scheduled_event_id=snowflakes.Snowflake(payload["guild_scheduled_event_id"])
-            if "guild_scheduled_event_id" in payload
+            if payload["guild_scheduled_event_id"]
             else undefined.UNDEFINED,
         )