I’ve noticed if the Great Wall gatehouse is destroyed I can repair like any other landmark but if the wall that it is built on is also destroyed, the gate cannot be repaired nor rebuilt. Not only do you lose this landmark but you lose all perks this dynasty provides. This has happended multiple times and it seems the issue starts once the wall it sits on is destroyed.
Desktop | Shot with GeForce - YouTube here is a clip showing what happens. Ability to repair goes missing at 2:09 apologies for the poor quality.
Not sure if this bug is similar to Chinese Great Wall Gatehouse Rebuilding Bug - #4 by urnotwirthy from October 2021