Hi Rapahel,
the issue is quite similar but not the same, actually.
I've had a look at the entries in T7EHS00_PERPROSC but I've found a few records regarding only two persons, while in the Cockpit I can't add to Planning list a dozen of people.
I've searched for a SAP Note, and found
http://service.sap.com/sap/support/notes/1067084
Still wondering if this depends on Exposure Group..