userassets.json contents:
[ { "type": "city", "name": "Big City", "dimension": 0, "chunkx": 10, "chunkz": 10, "radius": 100, "buildings": [ { "multibuilding": "center", "chunkx": 0, "chunkz": 0 } ] } ]

Done 3 tests. In each test I've set worldgen to Default, seed to 1234, then activated LC with GUI by pressing "Disabled" button, then "Customize", then set Rarity to 0.0, 0.000001, and finally 0.0001.

Expected: city being generated around x160 z160.

Result: nothing in the vicinity, and also not on any combination of x160, x-160, z160 and z-160. Subway is generated properly though.

The only suspicious error I've found in log:
[Server thread/WARN] [net.minecraftforge.common.ForgeConfigSpec/CORE]: Configuration file C:\Users\Nik-PC\AppData\Roaming\.minecraft\saves\Default 0_0001\serverconfig\lostcities-server.toml is not correct. Correcting

Also the mod is recognising userassets.json, as making a mistake in it will throw an error on game launch.

+1
I am also observing the exact same behavior. Was trying to create a central very large predefined city with very rare smaller cities.
I am on 1.16.5 MC and 1.16-4.0.10-beta LC Mod