Permission of a group with shared projects

There is a project P under group FOO, and I want to share project P with another group BAR which includes a ‘Guest’ member A. However, user A still has only Guest permission even the Maximum access level is set to Developer when project P is shared with group BAR.

The Maximum access level documents only permission downgrade, nothing about upgrade.
If the permission policy allows only for permission downgrade, not for upgrade, it it conflicts with the user permission documentation which says:

If a user is both in a group’s project and the project itself, the highest permission level is used.