Manage permissions being removed when a section is contentpage_edit ed

Give a user manage permissions on a section and zero or more subsections.

Now edit that section (i.e. contentpage_edit)

Press Save

The user's manage permissions on the page and all subpages have been deleted.

modules/navigationmodule/actions/save_contentpage.php calls:

navigationmodule::checkForSectionalAdmins($section->id);

Looking at the function in modules/navigationmodule/class.php reveals the comment:

// remove any manage permissions for this page and it's children

It doesn't look like the permissions are successfully granted after being removed following that comment and it's not clear why it even does that.