Steps to reproduce:

1. Open the game
2. Press Play Preview
3. If you haven't enabled the new UI yet, enable it by pressing "Switch to the New UI"
4. Go to the Realms tab (in the middle)
5. Press "Join a Realms server"

Observed result:
You are stuck on the panorama.

Expected result:
You are able to type in an invite code to join a Realm.

Note:
This is explicitly mentioned in the changelog:

We’ve introduced a brand-new Realms tab. It is found in the New Play Screen, but unfortunately there is an issue with actually joining a Realm this way! (Sorry!) You will have to switch to the old UI in order to join Realms in the Preview - we hope to have this issue resolved as soon as possible.