Internal user shares a SharePoint file with an external guest user that is already in our directory.
User gets the link and when authenticating is then prompted to set up MFA ('Your organization needs more information').
They click on 'Next' and successfully set up their MFA method of choice. They click next to continue and are taken back to the 'Your organization needs more information' screen. Clicking next takes them to a web page that says "You have successfully set up you security info. Click 'Done' to continue signing in". However, clicking 'done' (there is no other option) takes them right back to the 'Your organization needs more information' screen.
Azure shows that the users have successfully registered their MFA device. I've tried re-requiring MFA and revoking MFA sessions, but this doesn't fix the issue. I have tried different MFA options including MS Authenticator. This is happening on both real guest accounts and test guest accounts that I've created.
We are using Azure security defaults; we don't have any conditional access policies.
Why is this happening?