Unthreaded receipt is not returned in the next syncs

### Description

I'm investigating cases of stuck unread on EW.
When I'm sending an unthreaded receipt on a room with unread threads, my receipt doesn't appear in the next syncs.
Maybe is due to that the last event of the room is an user renaming

### Steps to reproduce

In a room:
- Unread threads
- Main timeline is read
- The last event of the room is an user renaming

I send an unthreaded receipt:

```
POST https://element.ems.host/_matrix/client/v3/rooms/!jSkNtsjcsXfCRJwGpp%3Amatrix.org/receipt/m.read/%24UVcMglbqEZkuH2fq5zzbH-gQGyIU3MKr83qdtWUJwHg
```
The next syncs don't include my receipt.

RoomId: !jSkNtsjcsXfCRJwGpp:matrix.org
Last important eventId: $UVcMglbqEZkuH2fq5zzbH-gQGyIU3MKr83qdtWUJwHg

### Homeserver

element.io

### Synapse Version

1.107.0

### Installation Method

I don't know

### Database

.

### Workers

Single process

### Platform

.

### Configuration

_No response_

### Relevant log output

```shell
.
```

### Anything else that would be useful to know?

_No response_

Start by reproducing the reported sequence against the Matrix receipt endpoint: leave unread threads, mark the main timeline read, ensure the last room event is a user renaming, then send the unthreaded receipt. Trace how that receipt is incorporated into subsequent sync responses; done means the receipt appears in the next syncs for this scenario.

Written by the indexing model from the issue text.