Changes in group forum permissions not saved

Problem:
Changing the group permissions of a group in a forum, where the group has a forum role, does not apply. After "Apply permissions" the success messages appears and the checkup shows, nothing has changed.

Reproduce:
Go to ACP -> Tab: Permissions > Forum Based Permissions: Group forum permissions > Group > Forum.
In "Forum permissions" > Advanced permissions > Tab: post
Change one value, like "increment post counter" from yes to no (or inreverse) and click on "apply permissions".
Then you'll get the "Permissions have been updated" message. If you go back to "setting permissions", the role ist still e.g. "full access" and the permission "increment post counter" has not changed.

This also affects "User permissions" and "Group permissions" in section "Global permissions".