It has been known for some time that TOTP 6 digit codes are easy to intercept. SMS Codes can also be intercepted, or gained via SSB7 vulns/ SIM jacking. This made things like Google Authenticator or Authy more resilient but certainly still quite vulnerable.
To intercept and exploit MFA in ProtonMail would absolutely trivial for a skilled single person to do. DNS poisoning + this github library would be all you needed: https://github.com/kgretzky/evilginx2
EDIT: replaced quotemark with asterisk