Thread replies do not local echo as expected

When sending an event as a reply to a thread using the default "chronological" pendingEventOrdering, I'd expect the reply to appear in the thread's timeline as soon as sendMessage is called. In actuality, with matrix-js-sdk@26.0.1, the event only shows up in the thread's timeline about half a second after the sendMessage promise resolves.

(I'd also be fine with the local echo appearing in the thread's containing Room, but no dice there either.)

Here is a branch of a repo that I think has a minimal reproducing example, although it's a little noisy due to many Thread.updates called – I believe related to #3463. (I don't think this is caused by the same bug as #3463 though, as this happens for me in threads with fewer than 11 events.)

Here are logs of me running this code:

Start with the minimal reproducing example referenced in the issue, then trace sendMessage with pendingEventOrdering set to chronological and the thread timeline update. Compare promise resolution with the delayed local echo; done means the reply appears in the thread timeline immediately when sendMessage resolves.