A Realm is shut down when its creator disconnects, but only if he is the one who initially launches that particular Realm session. If another member of the Realm is the first to launch it,any member (including the creator) can connect/disconnect without causing the Realm to shut down.

Steps to reproduce:

I created a Realm (and uploaded a single-player world to it).

I invited three friends to join.

If I am the first to connect to the Realm, all players are disconnected when I leave.

If I am not the first to connect (i.e, if one of my friends connect first), no one is disconnected when I leave.

Since I created the Realm, I theorize the game is treating it as a non-realm multiplayer instance when I am the first to launch it. Friends join the realm using the "Joinable Realms" interface (not "Joinable Friends), but are then disconnected when I exit.

If, however, someone else launches the Realm, it seems to be instantiated as a proper Realm. When that person leaves, the Realm stays up, and other players stay connected.

We have tested and verified this multiple times.