In effect, something su was doing broke whatever infosec voodoo systemd was trying to do to keep track of "sessions".

Part of this was what path that xdg variable got set to, so in their infinite visdom the systemd devs decided to clear it whenever su was run.

This in turn lead to various programs curbstomping user files as root (in particular Gnome ones, surprise surprise).

At this point in time it seems like one would do best to stay well away from anything related to Systemd, Gnome, Freedesktop and Fedora. The mixing of roles in those groups is downright worrying (i wonder how many times i have seen someone use a Gnome blog to port straight RH/Fedora news, even though the latter surely has their own blogging facilities).