I'm with confucius starting on chan china. When i want to build a megalith (culture city state unique improvement) on a great wall, i get 0 warning that the great wall will be replaced.
#Unique improvement bug
wary relic
amber quartz
I don't think this is a bug. The game never warns you when you are replacing an improvement with another improvement.
Which is to say, you're in the right place to bring this up. The game should be clearer about when something you're building is additive or a replacement.