Perhaps you could post which domain.tld you are using; should be no security issue since it is a Fastmail domain. I or someone else will repeat your test and see whether we can debug what is wrong. Perhaps Fastmail, which has a lot of domains of course, did not things set up right for one of them.
I also use the form
anything@mail2.domain.tld with domain.td being fastmail.com, and it works flawlessly for me.