Messages sent to a bare JID can be routed to resources with negative priority.

This was introduced with Message Carbons.

The problem is, that Openfire only routes to resources which share the highest priority. For Message Carbons this is counterproductive, because we want to route to all non-negative resources.

RFC 6121 (8.5.2.1.1. Message)

If there is more than one resource with a non-negative presence priority then the server MUST either (a) deliver the message to the "most available" resource or resources (according to the server's implementation-specific algorithm, e.g., treating the resource or resources with the highest presence priority as "most available") or (b) deliver the message to all of the non-negative resources.

AFAIK, Openfire only has logic for a), but not for b).

See forum thread for further information.