Read receipts move backwards or disappear after updating

### Steps to reproduce

1. Get a bunch of read receipts over several hours
2. Update/refresh the app
3. Read receipts move backwards or disappear

### Outcome

#### What did you expect?

Stability in other people's read receipts

#### What happened instead?


Clearly, Matthew has sent messages after his read receipt, so should have a receipt there. My other coworker's read receipt just isn't there, but was before I refreshed.

### Operating system

Windows 11

### Application version

Element Nightly version: 0.0.1-nightly.2023062201 Olm version: 3.2.14

### How did you install the app?

The Internet

### Homeserver

t2l.io

### Will you send logs?

Yes

Start by reproducing the issue in Element Nightly on Windows 11: collect read receipts over several hours, then refresh or update the app and compare them. Trace how read receipts are restored after refresh; done means existing receipts remain present and do not move backwards after the update.

Written by the indexing model from the issue text.