I've been using Sync for Lemmy for a year or so, but very recently (within the last day or so) links have stopped showing as read when I click on them and then go back. I tested this with a lemmy.world account and don't have the same problem.
I saw a post saying sh.itjust.works was updating to Lemmy v19.5 a few days ago. I'm wondering if maybe it caused a problem or if it's just me having this issue.
It's a known bug in Sync with Lemmy 0.19.5:
https://github.com/laurencedawson/sync-for-lemmy/issues/539
lemmy.world is still on Lemmy 0.19.3.
Ah that explains it. Thanks!