I was noticing terrible resolution speed, so checked messages (btw, "service bind9 status" provides brief, but decent-ish characterization of things), and noticed many messages indicating failing resolutions against IPv6 addresses.

There is a common pair of bits of Bind configuration:

a) One might set options to prefer not to use IPv6 addresses, so in /etc/bind/named.conf.options, change 

   listen-on-v6 { all;};

to

   listen-on-v6 { none; };

b) This was where SystemD got into the mix...

Pass the "-4" option when running named to use only IPv4.  This seems to be the highly significant bit.

On Debian-derived systems, the usual recipe is to add "-4" to /etc/defaults/bind

  --> OPTIONS="-4"

However, the SystemD config for Bind9 lives in /lib/systemd/system/bind9.service, and apparently there is recent change there that that configuration no longer reads from /etc/defaults/bind, so that to have named get the "-4" option requires altering /lib/systemd/system/bind9.service.  Seems like it needs to be altered directly.

This isn't necessarily a "SystemD bug", rather a packaging bug in Debian.  It still was fairly annoying.

I have yet to report it properly, probably deserves a bit more research, in case my diagnosis is off a bit.

--
When confronted by a difficult problem, solve it by reducing it to the
question, "How would the Lone Ranger handle this?"