I don’t email much, so i never paid much attention, but my messages don’t thread anymore. I thought that identical recipients would be grouped. I just keep getting ‘new chat’ contact requests for previously accepted senders. Subject lines not change. Thread works in k9 mail and Gmail.
When I reply, that single message gets grouped. Replies to my messages does not. Incoming just comes individually each time.
Android pixel 3a, delta.chat 1.8.1 via Google play store.
I found this old forum thread but didn’t understand the technical details of how group works.