Commit to protected (master) branch not possible as invited Maintainer

I invited a group to a project and gave “Maintainer” permission.
They can not commit directly into the protected master branch, although it should be possible as “Maintainer”. Users with “Maintainer” rights directly on the project can commit directly into the master branch.
Can anyone explain why?

Selfhosted GitLab EE (no license)

Thanks and have a nice holiday!

Anyone has an idea?