Hmm, tough to tell why attacking through the rivals list rather than rob/attacking would change something in the server.
From what I understand about attacks, the client sends an attack call to the server with your mafia id, some other info (cash, attack stats, skill points - most of this isn't even used) and their mafia id, the server calculates the battle result and any loot drops (items, money, RP) and returns a message saying what happened. The client should then process that info, collect the loot/money/RP (if you're in an attack screen) or show the loot/money/RP (if you're in their hood), and show the fight result screen.
So, my guess is there's some bug in the client that doesn't handle the return packet properly in that specific case (why, I have no idea). If I can trigger it I'll do some more research and send in a formal bug report. Would be nice to have CJ weigh in, first, though since it may be a known problem.
My wild guess is the client tries to put the cash/RP down somewhere, but it doesn't display since you're in your own hood (or someone else's). When the client is attempting to put it down in the victim's hood and you're not there to collect it, it disappears.