Not using the transport option clientCertFilename the certificates should be loaded from X509Store at class Apache.NMS.Stomp.Transport.Tcp.SslTransport method LoadCertificates.
But the collection store.Certificates is always empty, causing SelectLocalCertificate to always returning null.
This is not a big problem, because the connection is working using null. But maybe with an unexpected behavior if one is going to use the clientCertSubject transport option.
The reason is, the store must be opened (and closed):
Changing the lines in LoadCertificates to:
X509Store store = new X509Store(name, location);
collection = store.Certificates;
the collection is not empty anymore.