Pubsub parses all subscriptions for a user on a node, even though multiple subscriptions is turned off. This causes publishing an item to be very slow when many subscriptions are present.
I think the issue in this case will be what the behaviour should be. Which of a users subscriptions should be used if there are multiple and the multiple flag is turned off. Obviously parsing the same subscriptions over and over is the wrong approach.
It seems to me that the behavior is correct as implemented. Specifically, the current implementation expects the JID of a subscription owner as input, and returns a collection of all subscriptions that are owned by the given JID. Note that each subscription in this case will in fact have a unique subscriber JID, but they will share a common owner JID. This appears to be in compliance with the XEP-0060 specs (Multiple Subscriptions).