In my game, I decided to choose Khmer for medieval era since its unique district exploits all resources. This is particular useful for forests on rivers, which is one of most fertile tiles in the game. It is also the earliest available land districts that can exploit all resources.

However, I found Khmer's districts can only be built if your city is built exactly on river. The other inconsistent thing is, if you have a city on river, you can built Khmer's unique district on any rivers within tiles of your city, not necessary the same river as your city is built on.

I guess these inconsistency is probably a bug. Otherwise, I would like to know why the game designer makes this restriction.