The bug was, in essence, a hardcoded date for posts in the Micropub instance. It could’ve respected the date of sending if it was present, but it wasn’t.
That’s why it got the date from
now_iso() function, but the call was performed on launch instead of on post creation. It overwrote the same document over and over…
Thankfully, all my posts are saved in Git, so I’ve easily recovered my stuff. The feed now is in proper order.