Sync to furthest page read confusion

Done reading on the Mac. Close the app, move to Kindle.

Kindle offers to move me to furthest page read. I say okay.

But it's not the furthest page, it's an earlier sync. So I tell it to do furthest page read again. It says to "turn annotation backup on" in settings. I check, it's already turned on. I click sync/furthest page again. Same message.

So I go to settings, turn annotations backup off and the thing syncs to the correct page.

Curiouser and Curiouser.