Subversion repository manager appears unable to revoke permissions granted at a higher path

The Subversion permission manager appears to be unable to revoke a permission granted at the root path. For instance, given the default root permissions:

and a project FOO:

Clicking 'remove' on the users entry just causes the screen to refresh.

However, actually the change has taken effect! The FOO project's section in the authz file has changed from:

 [/FOO]
 @developers = rw
 @administrators = rw
 @users = r
 @_no-one-at-all = r
 _bamboo-svn-user = r

to:

 [/FOO]
 @users = 
 @developers = rw
 @administrators = rw
 @_no-one-at-all = r
 _bamboo-svn-user = r

The 5 minute cache on the authz file can further mislead an administrator that the change hasn't taken effect, when it has.

Also, it's now not possible to grant read access to the users group, because the UI thinks the group already has access.