Disposing/Closing of cloned sessions

we encounter a problem with the parallel usage of sessions. It doesn't matter if it's done with `Task.WhenAll` or `Parallel.ForEach`.
After Disposing a cloned session, the session is still alive (sleeping). And it doesn't seem that those sessions are reused.

A little test-script:

async Task Main() {
    var connData = await DbApp.Instance.ConnectAsync(DbApp.Instance.DefaultFactory, connectionString, true));
    using (var connection = connData.Connection) {
        var identity = connection.Authenticate(authString);
        using (var session = connection.Session) {
            //await Task.WhenAll(Enumerable.Range(0, 10).Select(i => Test(session)));
            Parallel.ForEach(Enumerable.Range(0, 10), i => Test2(i, session));

            // all sessions are still there
        }
        // all sessions are still there
    }
    // all sessions closed
}

async Task Test(ISession session) {
    using (var s = await session.CloneAsync()) { }
}
void Test2(int idx, ISession session) {
    using (var s = session.Clone()) { }
}

You can see the sessions in the database with

SELECT *
FROM sys.dm_exec_sessions
WHERE is_user_process = 1
AND host_name = '...'

Disposing the connection is done relatively seldom - thus I would assume that the DialogScript ADS_AssignADSGroupsToITShop is affected as well.
In the object log I see as many "Closing session"- as "Opening session"-entries - but it seems that there's no closing at all.

So the questions is: how to use session.Clone in a way, the sessions are closed/reused after usage?