Yes, in an earlier HN thread I remember belatedly tracking down and being disappointed by this behavior. It wrongly led me to believe for a moment that the A record itself had been removed!
As a quick warning to other Linux users, if you're using a Linux system you may well not be able to resolve single-component DNS names with your OS-default DNS settings at all, even when they're valid in the global DNS.
For me, before changing that setting, lookups for ai. were failing. After changing it, they worked. I also had to make that OpenWRT change mentioned above since my router is running OpenWRT.
As to the question of whether or not lookup for ai. should work even without that setting... I dunno. Maybe it's just down to the way the systemd-resolved maintainers interpreted the spec?
As a quick warning to other Linux users, if you're using a Linux system you may well not be able to resolve single-component DNS names with your OS-default DNS settings at all, even when they're valid in the global DNS.