I was in a skirmish as French vs Chinese (Hardest AI) and I noticed my walls being repaired by the Chinese “Extra Materials” technology for their outposts/towers. There was no repair icon above the wall segment, but it was definitely being repaired.
This was supposedly fixed in patch 8324. I never tested it then, so I don’t know if it either regressed or was never fixed.
Build: 5.0.9369.0