I have faced the same issue as others on this thread: many single-use code emails, but no unauthorised log in attempts when I reviewed the access history.
Why the problem was occuring:
My primary email address was listed *on another Microsoft account* as a recovery email/method to receive a secure code to log in. I had forgotten that account existed, but once I logged into that account I could see many, many unauthorised attempts to log in (from a VPN presumably, since there were connections from everywhere).
Solution that worked for me:
Work out what old account I had set my email as a recovery account for. Log into that account and removed my email.
Recommendation to Microsoft:
- include information of which account is generating the single use code email
- consider ways to improve security, since as per a previous post of mine, I have concerns that bad actors could brute force a 7 digit number 'password' with enough attempts/email addresses.