I have run into this bug three times, all on hardcore mode. When I loaded my world I spawned in a blue void. I was not falling and I couldn't see or interact with anything. The first time it happened I ran the world without optifine and the bug was fixed. However, the second time the blue void still appeared after loading the world in the latest installation (1.15.2). The void is the only thing I see no matter how many times I reload the world or relaunch the game. It doesn't appear to go away with time, but I haven't left it for more than 10 minutes. It also doesn't crash the game.