Unit transparency ignores overrides

[TV4Fun]

When unit transparency and overrides are enabled, the background behind a unit seems to be rendered in the plain graphics font, ignoring overrides. This is very obvious using a graphics set like Phoebus. See the screenshot here for an example. Most of the bridge is rendered with unique graphics, but the part the cat is standing on is rendered with the base graphics font.

[AuJaeger]

I'm also having this issue.