Siskin does not generate push notifications for incoming messages when the client is in the background (disconnected). Those notifications are generated by your XMPP server and Siskin just displays them. Notifications that are generic "New message!" do not contain any message but are sent by your XMPP server. If your server sends notifications with content/message, then the proper message is displayed.

We are aware that some XMPP servers send notifications for any incoming XMPP stanza or message (even if there is no body in the message), but that is in our opinion violation of the XEP-0357: Push Notifications and should be solved on the server side.

In future versions we may decide to filter notifications on the client side, however, I do not think this is how it should be done.