Your config looks fine, not sure why it is only reading from a single one.
{ find: "objects", filter: { _key: /user:/i, username: "admin" }
This doesn't look like a query we do in core nodebb, do you have some plugins that run custom queries? This doesn't use the indexes properly should probably be
{ find: "objects", filter: { _key: /^user:\d+$/i, username: "admin" }