Simply put: While in fullscreen, on the Windows 10 Edition (not java), if you alt tab out while in a realm you are kicked. Minecraft build if you need it: 4209675

How to replicate:

1. Go onto a realm.
2. Make sure you're in fullscreen.
3. Alt tab out and alt tab back in, even within 5 or so seconds.
4. Wait for about 10-15 seconds, as it looks as if you are still in the realm, but network connection is lost or something of that sort.