Thanks for the heads up Paul – I think this is definitely related to Hummingbird re-triggering pings when the site is re-published, when a new post (or comment?) is added. I’m only running cron tasks at 3am UTC, which ties in with the timing you noted before.

I did start having a look into the webmention code to debug it, it gets a bit confusing when replies to comments get added in as well, but it looks like I can keep track of when pings are sent. I’ll ask around to see if it’s a known issue already too.

Apologies!