Reports of events in rooms where the reporter has been invited to (but not joined) are accepted by the API, yet hidden when querying reports until the reporter joins

### Description

I understand that the spec says a user must have joined a room to report any events in it. When testing it (originally meant for responding to a different issue...) I noticed this odd behaviour.

### Steps to reproduce

1. User receives invite to a room
2. User sends a report on the inviting state event. (Assume the event and room IDs are provided and correct.)
3. Server returns 200 `{}` to user
4. Admin tries to query reports (I'm using synapse-admin to query all reports on a homeserver)
5. Server does not return the report to admin
6. User joins the room
7. Admin tries to query reports again
8. Server returns the report to admin

### Homeserver

bus-hit.me (private)

### Synapse Version

1.111.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

It is PostgreSQL, but I just used whatever matrix-docker-ansible-deploy suggested. I just updated it today.

### Workers

Multiple workers

### Platform

Ubuntu on Oracle Cloud; matrix-docker-ansible-deploy is used.

### Configuration

None relevant.

### Relevant log output

```shell
None visible.
```

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

My personal wish is if report function allows reporting the invite event without the recipient joining the room, but that's a spec issue.