It’s possible that your work has problems with spam and has thus set up their own spam filter and that filter might be more aggressive.
There are also ways to change the spam filtering in m365 mail.
It’s also possible that your work hasn’t setup mail properly and protonmail is the one that rejects the mails. We had an issue at work where OOF auto replies didn’t work to external gmails (and probably others) because Gmail rejected the emails (I had confirmed that Gmail was the one that rejected it via the email traces in exchange). That wasn’t Microsoft’s fault.
I can send stuff to my proton mail without issues from two different Microsoft 365 mails.
One missed email should not be interpreted as malicious intent. It’s also honestly quite likely someone just misspelled or something.