Could this not have been a case of a series of private apparitions that were later recounted by the viewers to Matthew when he wrote his Gospel, events of which Pilate was unaware of?
I guess that depends on your NT perspective, literal vs inspired tales. Taken literally, the holy city filled with the risen dead was quite the event,and surely would have impacted more than just the Jews there, and would have been quite the surprise to the Romans of the time, who would have had to confront the risen dead. After rising, did they return to their graves? Were they lifted up? Did they Romans chop them to bits?
If not literal, I suppose it could have been private apparitions or mass hysteria, but what does that say about the other events that occurred?