It's not. I wrote about this a couple times. Here's how it works: the personal spam filter kicks in after having trained on ~100-200 spam emails. You need to mark them as spam (you might want to use a filter initially if they all have a common pattern) and delete them regularly from the spam folder. Spam training kicks in only when you a spam message is deleted (to avoid training on false positives)

I went through the same phase when I first moved to Fastmail, and this is what customer support explained after some back-and-forth. I don't have spam in my Inbox any more (despite receiving ~20 spam emails per day)