This has been happening with every wall at Fare for the past hour. You can just stand in the courtyard and watch the whole keep flip back and forth.
The health of the walls jumps constantly between 100% health with 200k HP and 65% health with 305k HP. The postern houses and construction equipment (cranes/scaffolding) appear and disappear accordingly. The flips occur quickly, about every second. Occasionally it's possible to repair the walls, but then it immediately goes back to flipping and being unrepairable.
It seems like this is a bug with the new upgrade/degrade system.
Keep gates do not upgrade or deteriorate at all, and the health numbers seemed to have sky rocketed since the update with the outer doors typically having 225k hp and the inner doors having 375k hp. Also, I saw a wall today with 620,000 hit points...that's just ridiculous. It makes me wonder if the stone upgrades are somehow stacking unintentionally.