ipv6 nmap breakage under 6.5-STABLE ?

Hi,
[Cross-posted to misc & ports as I'm not sure if there's a bug in
software or in wetware.]

I'm trying to run nmap (from ports) on 6.5-STABLE but am getting an
ungoogle-able error message every time:

root@bgpmirror:~# nmap -Pn -A -n --top-ports=100 -6
2620:132:300e:700::113
Starting Nmap 7.70 ( https://nmap.org ) at 2019-07-22 09:43 CDT
sendmsg: Message too long
sendmsg: Message too long
[...continues like this for a really long time...]

Interspersed semi-randomly (not always at the same point) I also see
mixed in there a single occurrence:

sendmsg: Message too long
sendmsg: Bad address
Unable to send packet in probe_transmission_handler: Bad address (14)
sendmsg: Invalid argument
Unable to send packet in probe_transmission_handler: Invalid argument
(22)
sendmsg: Message too long
Unable to send packet in probe_transmission_handler: Message too long
(40)
sendmsg: Message too long

Performing a similar scan on an IPv4 address works as expected. This
appears to be a v6-specific problem.

Known issue?
Workarounds?

Thanks,
-Adam