Note to future me: when Publii refuses to complete syncing this may help — Website not syncing completely - Publii Forum:

remove file files.publii.json from your server and the sync website again (it will upload all files of your website).

It definitely worked today. Whew!

@pratik I use it for a community news site I run. Mostly it works well enough and means I can avoid WordPress. I compose in the app on my Mac then sync to get the posts online.