my minecraft server biomes are transforming to ocean biome

i was in my minecraft server that its made with aternos and i was playing with my friend and for some reason i stopped the server and when we came back into server the plains biome that we was in it was changed color and it was ocean biome and when i closed the server again and when we came back into the server another part was changed to ocean ( i was uploaded the c&c preview datapck on the server ) . i was having this problem on 21w15a snapshot to.