As defined in src/main/resources/data/mining_dims/dimension/mining_dim.json, plains should be generated too. However, I have been unable to locate any – not via commands or by manually searching. If I'm incorrect in thinking this is not the intended behaviour, then I apologise.

It is supposed to be both desert and plains, I will look into this. Might be an issue with the new biomes api and data packs.

Fixed in v1.2.40