Discovered while testing MC-169008.

So, I spawned at about 200 blocks away from the main island. But the bedrock fountain didn't appear and normal end music was playing until I came close to it. After that, the fountain appeared, the fog abruptly appeared and the end music stopped abruptly and the boss music replaced the music. When I came away a bit, The fog abruptly disappeared and the normal end music started playing. Those changes are 160-190 blocks from the center fountain.

This bug is with us at least since 1.13.2