Back pagination can stop working

No more details yet but we have an internal report that pagination can stop working. It worked again the day after.

The client got a 403 error:
```
2022-01-16 21:31:51.213 Riot[17714:1509046] [MXHTTPClient] Error JSONResponse: {
errcode = "M_FORBIDDEN";
error = "Host not in room.";
}
```

The report names only back pagination and the MXHTTPClient 403 log, with no file, test, or reproduction steps. Start by tracing the back-pagination request path around MXHTTPClient and investigating the intermittent “Host not in room” response. Done means identifying why pagination stops and verifying that it continues correctly after the failure.

Written by the indexing model from the issue text.