A case of zombie notifications

### Steps to reproduce

Unclear

### Outcome

#### What did you expect?

Rooms that I've read will stay clear of notifications.

#### What happened instead?

I have an encrypted DM which reliably produces a gray "1" notification badge every time I start my client. There are no threads in this DM, I am not using private read receipts, and I also have the Threads Activity Center enabled.

The latest events are from June 4th, 2023, and I can't see anything suspicious about them: they consist of four `m.room.message`s, followed by a reaction from the other user, and finally another `m.room.message` from the other user.

Digging through the room object, I see one thing out of the ordinary compared to my other DMs: I appear to have an unthreaded receipt, which points to the last `m.room.message` that I received prior to June 4th.


(`$Kl7c...` is the most recent event in the room, and `$nidq...` is the message received prior to June 4th.)

I'm happy to provide more information, I just don't know what to look for beyond this

### Operating system

NixOS unstable

### Browser information

Firefox 122.0.1

### URL for webapp

develop.element.io

### Application version

Element version: 92c831094d11-react-35ad92bf540a-js-74c109adacd4 Crypto version: Rust SDK 0.7.0 (fac36bc), Vodozemac 0.5.1

### Homeserver

Synapse 1.99.0

### Will you send logs?

No