Can't join a room as a guest if email is a required 3pid for registration

[matrixbot]

This issue has been migrated from #4146.

---

Description

I cannot join a room as a guest if email is a required 3pid for registration.

Steps to reproduce

• Ensure the following is in homeserver.yaml

    enable_registration: true
    registrations_require_3pid:
        - email
    allow_guest_access: true
    enable_registration_captcha: true
    recaptcha_public_key : "REDACTED"
    recaptcha_private_key: "REDACTED"

• attempt to join room as guest

I expect to be able to join the room as a guest, but I get No appropriate authentication flow found.
If I remove email from registrations_require_3pid I am able to join as a guest successfully.

Version information

• Homeserver: Self-hosted

If not matrix.org:

• Version: v0.33.8
• Install method: docker
• Platform: Running in Kubernetes behind ingress controller (reverse proxy)