Unread indicator never clears for some thread

### Steps to reproduce

I don't know how to reproduce.

### Outcome

#### What did you expect?

The unread indicator for any thread to disappear when I open it.

#### What happened instead?

The unread indicator for a specific thread never clears.

Some observations that may be relevant:

- The thread is in a DM room.
- That room has two threads.
- That thread is the more recent one.
- The last message in the room is deleted.
- The thread is started from the message that is previous to last.
- The earlier thread is started from the message just prior to that.
- The room is encrypted.
- The thread doesn't seem to have any messages with reactions.
- The thread has some voice messages.
- The last message in the thread is a text message by the other user.
- The one previous to that is a text message by me.
- The last message in the thread is marked as seen by the other user.
- The DM room was created by the other user.

It is unlikely that thread will ever see more messages, if that helps.

### Operating system

NixOS 24.11.20240920.a1d9266 (Vicuna) x86_64

### Browser information

Firefox 130.0.1

### URL for webapp

https://app.element.io/?updated=1.11.62#/room/!zgqAHuqAyuGAtOuHMm:matrix.org

### Application version

Element version: 1.11.77 Crypto version: Rust SDK 0.7.1 (c8c9d15), Vodozemac 0.6.0

### Homeserver

_No response_

### Will you send logs?

Yes