The bug

Using arrow keys in Minecraft Realm menu allows you to connect to an expired/closed realm (but fails)

Base issue:

The Play button doesn't update to non-greyed out or greyed out when using arrow keys, but does when using cursor

Priority: Minor

Reproduction

You should be invited to an open realm and a closed/expired Realm. Select the open Realm, and use the arrow keys to go down to the closed/expired Realm. Pressing enter does send a 403 error instead of doing nothing.
This can be an issue for the other way; select an expired/closed Realm, then use the arrow keys to go to an open Realm. This will have the button grayed out still.