Pillaged Palace

[Miserable Old Git]

Something I've not seen (or maybe just not noticed) before: after my capital was nuked (regular nuke not thermo), and everything has been cleaned up after it (all districts, buildings, and tiles fully decontaminated and repaired) it still shows that the Palace is pillaged. Screenshot attached (not wholly clear, though the city centre is shown with the Pillaged search; a mouse hover over it specifically shows "Palace (pillaged)" but this can't be captured in a screenprint).

There seems no way to repair it, and a google search yields zilch.

Any ideas?