Scrollback shows mxids and blank avatars after a certain point

*This issue was originally created by [**@0x1a8510f2**](https://github.com/0x1a8510f2) at .*

### Background information

- **Dendrite version or git SHA**: d5978d98fda8a31918973a2481e62eafcec9f80f (but bug noted in every version so far)
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: Yes
- **`go version`**: n/a
- **Client used (if applicable)**: Element Web / Element Android

### Description

- **What** is the problem: When scrolling back in the timeline, messages after a certain point do not show the sender's displayname or avatar, only their mxid.
- **Who** is affected: Users of the homeserver.
- **How** is this bug manifesting: Missing user avatars and displaynames in scrollback.
- **When** did this first appear: Since I've been using Dendrite.

### Steps to reproduce

- Open any room
- Scroll up

Reproduce the issue by opening a room and scrolling up in Element Web or Element Android against the Dendrite monolith using Postgres in Docker. Compare the timeline responses before and after the point where sender display names and avatars disappear, then trace the relevant Dendrite timeline and user-profile handling. Done means scrollback consistently shows the sender display name and avatar instead of only the mxid.

Written by the indexing model from the issue text.