OIDC: Custom additional claims not present in userinfo

An access token with scopes openid, profile, email and groups is used for the following request:

curl -v -H "Authorization: Bearer TOKEN" http://localhost:8000/oauth/userinfo/

which returns

{"sub": "1", "preferred_username": "felix", "given_name": "Felix", "family_name": "", "profile": "http://localhost:8000/django/persons/1/", "email": "felix@example.org"}

groups is missing in the response.