I have two possible Volcano biomes in registry. One from fossils; the other from WildNature (MC 1.12.2). However, I am using BiomeTweaker to replace the WildNature biome with the Fossils biome for Volcanos globally. I don't think that any of that is relevant, but it could be.

I have discovered this "floating" volcano world-gen today, which needs fixing. It looks like the volcano edge stops at the change of biome (still way above the ground level), rather than generating smoothly onwards to encroach on the neighbour biome. Could this be caused by BiomeTweaker? Or is it a Fossils issue?