Playing a sky block world in BDS 1.17.32.02, I've found recently that when falling into lava, it causes the client to freeze (MCPE in this case).  Upon attempting to relog, the player can log in, but is presented with only their hotbar (which they can interact with), but they appear to be in the void (though taking no damage).  Any attempt to rotate/move causes the client to freeze.

Upon inspection of the world using MCC ToolChest PE, I found that one of the player's rotation was set to NaN (JS for not a number).  Changing this to a valid value (i.e. 90) allowed the player to log in properly again.