Chat scrolling up to read messages

### Steps to reproduce

1. Opening private (People) chat room
2. Chat scrolls up to already read messages, clicking to scroll down, doesn't scroll down all the way but need to manually scroll to the bottom
3. After going out and again in the lobby, then it fixes itself

### Outcome

#### What did you expect?
On opening lobby, it scrolls to the actual unseen messages.
#### What happened instead?
It scrolls up to already seen messages and thinks they are unseen.

Until logic is fixed for seen messages, option to always scroll to the bottom would be welcome.

### Your phone model

iPhone 12

### Operating system version

15.4

### Application version

1.8.3

### Homeserver

matrix.org

### Will you send logs?

Yes

Start by reproducing the issue when opening a private People chat on an iPhone 12 running iOS 15.4, using the steps in the report. Investigate why the lobby opens on already-read messages instead of the actual unseen messages; done means it opens at the unseen messages or reliably at the bottom without manual scrolling.

Written by the indexing model from the issue text.