pouchdb: Filtered live replication - Many 404 errors

I’m doing a live filtered replication, like this:

    const sync$ = pouchStore.sync(remoteDB, {
        live: true,
        retry: true,
        filter: 'userView/by_userId',
        query_params: {userId: '1234'}
    });

I’m seeing many 404 errors appearing in my log, occurring according to the default back-off times.

GET http://gateway:5984/data/_local/WrCyesQFsZ6HR3n2FoCEAA%3D%3D? 404 (Object Not Found)

The data does, however, sync correctly to the browser, albeit with the 404s appearing continually. I’m thinking that maybe the errors are related to the ‘push’ side of the sync.

If I remove the retry option, the 404 appears once and the sync is immediately aborted with this error:

If I run without the filter, I just get the usual 404s with the reassuring ‘this is normal…’ log message and sync works correctly.