I was playing with my mini modpack with 10 mods and noticed,what some mod was creating biomes made of diamond blocks. I created 20 worlds while deleting one mod each time and AlexsMods was the mod who done that. Playing on 1.18.1. (that file called 1.15.1).
Not sure if this bug related to AlexsMobs mod,but it was the only mod installed when this biomes generating.