Both on mobile and windows 10 I have run into this issue I believe it's an issue with bedrock server software that's why it's happening. So sometimes players play totally normally then leave the server and come back later and when they come back they are spawning in a random place in the air and falls to their death with no command issued or no error issued in the console but bedrock server software never issue errors what I know. Also I have seen players reporting this issue in Minecraft rating and stuff in Google play store