Defualt World in Superflat World

I once made a superflat world many years ago (V1.14.0 - 1.14.4), but in the distance, there was a defualt world. I came closer and some lava of the "caves" flowed down the edge because it was cut of by a chunk. it was on an android tablet. it was on mcpe. i saw a swamp biome with a witch hut. i ignored it and blowed something up with tnt, spawned some wolves and experiment with the acacia wood. but after re-opening the world, it was still there, the world in the world...