QueryAsync, non-buffered, with mapping throws "No columns were selected"

This works as expected

await connection.QueryAsync<T1, T2, T1>(
    new CommandDefinition(sql), 
       (t1, t2) => // some mapping func
);

But this throws "No columns were selected" when enumerating the results:

await connection.QueryAsync<T1, T2, T1>(
    new CommandDefinition(sql, flags: CommandFlags.None),  // effectively turning buffering off
       (t1, t2) => // some mapping func 
);

The sync version works both buffered and non-buffered:

connection.Query<T1, T2, T1>(
    sql,
    (t1, t2) => // some mapping func,
    buffered: false
);

Note that the connection is NOT closed at enumeration time.

Tested using Dapper v. 2.0.30 on Postgres database using NpgSql v. 3.2.7.
Upgrading to NpgSql v. 4.1.3.1 still throws an exception, but message is now "The reader is closed"

The QueryAync<T> version (no mapping func) also works as expected non-buffered. We ran into the issue as we needed to add a join and a mapping function to existing code.

Start by reproducing the async multi-mapping case with CommandFlags.None against PostgreSQL, then compare its enumeration with buffered and synchronous Query behavior. The fix is complete when mapped rows enumerate successfully without a "No columns were selected" or reader-closed exception while the connection remains open.