Plone.api.user.get_users(group) : Feature or Bug? Not all users returned

I noticed that if I use

api.user.get_users(groupname='Manager')

Only user that are 'ticked' are returned.
Feels a bit strange to me. Is it supposed to be like that?
(in other words: user 2 and 4 in screenshot are returned)

Is this a group in group situation?

Not sure what you mean by 'group sitution'.

If I add a new user and add him/her to a group (in the add user 'schema'), it will not 'list', but if I add a new user and then go to user profile and add him/her to a group it will show.

Can check later if it is the same if I add user 'from group' (instead of adding 'group' to 'user'