Indeed! This is why I stopped using it. I love Fastmail, but who knows if I feel that way in 5 years. The entire point of Fastmail + own domain is never being locked in again. Using subdomain addressing locks you in once again.
I'm with @rb666; Don't rely on it as most will support plus+ addressing but not the the fast mail subdomain addressing as I am now in the process of migrating to Migadu.com and I need to go and unsubscribe and resub using the plus+tag. It's a PITA... lesson learned, stick with best industry practices even if there is an easier method because you'll thank yourself later.