At the intersection between a Soulsand Valley biome and either of the nether forest biomes, basalt pillars can mix with the generation of large fungi (as seen in the screenshot). The basalt pillar ends up replacing blocks in the large fungi in a very unnatural way. The expected behavior is that the basalt pillar would either end on top of the large fungi, or it would end on the nether floor with the large fungi generating around the basalt pillar.

This screenshot was taken on the seed -769589120886783917 in the nether dimension at the coordinates (-869, 80, -166).