I am unable to join a friend's world and they cannot join mine. The iPad that uses a different account says 'Unable to join world' whenever any device attempts to connect over LAN, but works fine otherwise. It also cannot cannot join the worlds of the two later mentioned iPads in Terraria, but they can join each other's.

The iPads that can't see the world in the 'Friends' tab use the same Microsoft account as the computer, but I'm pretty sure that an account could be used to join the same account's world on a different device.