I reported an issue to PowerDNS, and they tell me:
When I check the version of PowerDNS you're currently running, it looks like 4.0.4. Have you upgraded since we began the discussion, or have you been running 4.0.4 all along?
$ dig +short version.bind chaos txt @ns1.zxcs.nl
"PowerDNS Authoritative Server 4.0.4 (built Jun 22 2017 20:14:47 by buildbot@c1b965951e5b)"