The response:
I looked into this further and it sounds like this is expected behavior as you are using application-level MFA for the OpenID Connect application. Unfortunately, app-level MFA is incompatible with self-hosted sign in pages, which explains why you are being redirected back to the Okta hosted sign in to complete the process.
I’m curious as to why it worked for you previous, but my guess is that you had an Organization-level Sign On policy in place instead that prompted the user for their factor. One solution to work around this issue would be configuring an org-level MFA to apply to groups assigned your application, a group that you could then use for application assignment.
You may also want to submit a Feature Request for this setup to be supported on the Okta Ideas site, which you can access by logging into your Okta Admin Console, clicking on the link for Help and Support (which takes you to our Support site), and then navigating to Products > Ideas. Once ideas are submitted, other Okta admins will have the ability to vote on them to help our Product team prioritize requests; additionally, you will be able to monitor the potential for future enhancements there.