Read markers jumping around

### Steps to reproduce

1. Where are you starting? What can you see?

In a large room with lots of people reading messages.

### Outcome

#### What did you expect?

I would expect the read markers to remain still, especially when the number of people who have read a message does not increase.

#### What happened instead?

https://github.com/user-attachments/assets/ebe5c124-ce0b-4495-b3ba-2326668643c0

The read markers move around, even when new people's icons are not being added.

### Operating system

Debian 13

### Browser information

Firefox 152.0.6

### URL for webapp

develop.element.io

### Application version

Element version: 707fbd960a11-js-040fc6026bef Crypto version: Rust SDK 0.18.0 (6fe03c1), Vodozemac 0.10.0

### Homeserver

matrix.org

### Will you send logs?

No

Start by reproducing the issue at develop.element.io in a large room with many readers, using the listed Firefox and Debian environment. Observe the read markers while the number of people who have read a message stays unchanged; done means the markers remain still rather than moving around.

Written by the indexing model from the issue text.