Reported reported many many times times 😉
Nothing can be done about it until Flarum’s core is fixed, sadly.
I haven’t looked to see if it got fixed, but this sort of fix is already present in some situations, so fingers crossed it isn’t too long until a fix is made.
The issue is a combination of client and server. The fix is for the client to send the ID of the last thread in the list, and the server starts sending data from that thread onwards. For now, it just starts from the number of the last viewed, so if a new thread is made, all the existing threads get bumped down by 1 or more.