A nearby independent power at the start of Exploration turned hostile, and started attacking the wonder. I was already in the process of allying them, and they were no risk to the city itlself, so I didn't bother intervening. However, they did manage to bring the walls down on the wonder, and since then, it's impossible to restore it. The walls don't heal by itself. It offers a repair project, but trying to use production is as busted as with any other ageless building, and spending gold will temporarily restore it, but the health of the walls remains at 0, so the next turn, it will go back to pillaged. I've done the latter twice (as I wasn't convinced I actually did it first time around), and all I got out of it is -344 gold.