Note that the syntax for dig uses a @ before the server, ie
dig fail01.dnssec.works @127.0.0.1 -p 5335
That said, I also have intermittent results from that server (example from March). It will often time out the first time and then work (as in SERVFAIL as expected) the second time. I set up Pi-hole and Unhound for a couple of friends and saw the same behaviour on their setups too.