Can anyone help me troubleshoot why some visitors are taken straight to a Miro board via a shareable link, whereas others are asked to create an account?

I just ran a virtual session, where 80% of the visitors could access the board via the link (some of which already had a Miro account, but many of whom didn't). I am a paid user, and set up the sharing settings as per the instructions.

I have another workshop later today and would really like to avoid the same issues!

Here is the board in question: https://miro.com/app/board/uXjVPiKMjcg=/?share_link_id=6425333