That may be the case, but I've also had this issue with an actual NA 3DS that I streetpassed (when trying to recreate the problem), and the only streetpasses I've ever received for SMT4 were from Japanese consoles.
I've got StreetPass Relay to work using my Android, but I've noticed that I'm not getting SMTIV hits even if I use the SMTIV/FE/EO4 MAC address. I think it's because SMTIV has region-locked StreetPass since the only time I did get it was on another MAC address from another Japanese 3DS. (If...