The bug

There is no specification in the vanilla world gen settings that lava lakes should generate stone. However, generation of lava lakes will always place stone, implying this feature is hardcoded. I've tried adding new lakes with blocks other than water or lava to see if something else was responsible but these had no effect.

How to reproduce

1. Download the attached data pack
2. Teleport to the added biome
3. Note how all lava lakes generate with stone
4. Download the vanilla world gen settings from this video https://www.youtube.com/watch?v=xcQhAth2bpQ and note how replacing stuff with stone does not appear to be mentioned at all in the files within associated with lakes