I fixed the city border not updating to the new color after a settlement changes owner by capture, peace deal or integration.
I also fixed some/most/all (?) cases of single tiles showing their own border instead of being part of the city border for that settlement. I'm not convinced yet that I fixed all possible cases. Therefor this version writes a lot of extra info to the UI.log, which helps me troubleshoot any leftover border mismatches.
If you still find a mismatched border at some point, I would be very grateful if you would send me a screenshot with the spot, UI.log (zipped) from that moment and an (auto)save from one or two turns before it happened.