Additional Details:
I was able to replicate the issue on the "Rob Skyway City Bank" mayhem mission issued by Lord Weitzle in Port Oaks. I ran the mission +3/6 (which I don't think necessarily makes a difference). Once Longbow entered the elevator some NPCs became invisible and untargable. However, you could still see their weapons, they still took damage by way of damage auras, and they could still attack. However, I've seen it in the past with PPD l. This occurred on the Excelsior server.