Take a look at the query that this is generating - Ecto.Adapters.SQL.to_sql/3 will give you the actual SQL that gets sent.
The usual culprit in cases like this is that LIMIT applies to the total number of rows returned, but when preloading in a single query there will be one row in the result per OrderProduct; if there are orders with more than one product, fewer than 5 orders will be returned.
