Broken IPv6 network — multiple devices sending IPv6 RA

Back in early June I started getting emails from updown.io warning me of IPv6 ping issues to my CHR edge router. I didn’t notice anything wrong, and everything seemingly worked just fine, so I muted the alarm and did summer stuff instead. Lately I’ve noticed that wget and curl, when IPv6 was used, would often time out — not always, about half the time.