Although this seems like an intended feature (since cave biomes are still biomes and you choose the "single biome" generation), I argue it's a bug and needs to be fixed. Cave biomes add so much more exploration into the game and shouldn't be removed due to choosing a "single biome" world generation for the overworld. The single biome does not affect generation of mineshafts and strongholds, so why should it prevent an ancient city from spawning?

I've attached screenshots showing how these biomes/structures (in the case of the ancient city) do not appear in "single biome" worlds I have created (I tested it for desert, snowy slopes, plains, and frozen river).