IPv6 weirdness

We have a weird rise in incoming IPv6 traffic for 12 hours a day between 1900 and 0700 UTC. It's an almost sudden rise so it doesn't seem to be due to a certain geographic region waking up. According to tcpdump, it is directed at port 443 but nothing out of the ordinary turns up in the nginx logs. There is no discernible rise in outgoing traffic so it doesn't seem like a scraper that activates at certain times of the day. Here are some graphs demonstrating the traffic:

(The final spike at the end is expected -- it's one of our cron jobs running a big task.)

We first enabled IPv6 in September 2017:

The incoming traffic started in October 2017:

And it's occurred every day since: