Not sure if this is specific to Bedrock Edition client or the server side, did some searching online and didn't find much specific information. 

It seems that if Minecraft Bedrock Edition is running in Fullscreen mode and you alt+tab out (to maybe check the Wiki or whatever) it immediately disconnects from the BDS. Not even a few seconds delay it is just instant. Makes wanting to play in Fullscreen and still be able to do console commands on the server itself impossible since you can't tab out and then do a command on your own player since you will be disconnected at that point.