Action_session_start not getting triggered

I’m trying to override action_session_start to fill some slots on session start. I’ve just copied this example: ActionSessionStart to my actions.py and just changed the fetch_slots method. If I run rasa run actions --debug I can see, that this action is registered:

2020-11-24 17:19:19 INFO rasa_sdk.executor - Registered function for ‘action_session_start’.

But neither on my first message, nor on manually triggering /start_session the action is getting triggered.

It does work with other actions. Do I need to change something else, or should overriding the action be enough?

Thanks

Did you also add the action action_session_start to our domain?