Message previews not always initially present and don't reliably populate after visiting the room

### Steps to reproduce

1. Find a room that hasn't had messages in a long while (my examples have plenty of recent display name changes however, unsure if important) such that the Room doesn't show a message preview in the room list
2. Visit the room
3. Message preview then appears for some rooms but not others in the same situation

### Outcome

#### What did you expect?

* Ideally - message previews always present for all rooms without needing to visit any of them
* At least - for any room missing message previews visiting the room should be enough to populate the message preview

#### What happened instead?

* Rooms without messages in a long period of time don't get message previews
* Visiting a room and scrolling back to the last message doesn't reliably cause a message preview to be populated

### Operating system

Ubuntu Linux

### Application version

1.9.9

### How did you install the app?

Deb

### Homeserver

banzan.uk

### Will you send logs?

Yes

Reproduce this in Element Web 1.9.9 on Ubuntu using a room with no messages for a long period, checking the room-list preview before and after visiting and scrolling to the last message. Trace the room-list preview population flow and compare rooms that do and do not update; done means previews reliably appear without visiting, or populate after visiting the room.

Written by the indexing model from the issue text.