Back in MCPE-31064 it was mentioned this was an issue when playing in fullscreen mode.

That issue was resolved as "fixed" after a workaround was provided of "Run minecraft in windowed mode" which is not accurate. Disconnecting upon minimizing in full screen is absolutely a bug and should still be investigated.

I can provide any additional reproduce details but its pretty straight forward and easily reproducible.

Windows 10, Play in full screen, alt+tab away from the game so that it minimizes, return and you've been disconnected from any server you were connected to.

Let me know if any additional details are needed. Thank you!