This is probably a WAD but not WAI behavior. All the usual conditions apply - no DB modding, period as decimal, etc. etc.
Recently had an NPR decide they hated me and they informed me of this by blowing up a few of my unarmed scout ships, JP monitors, and so on. Shortly after this, before I could retaliate in kind, I received an event message on the construction tick that the NPR now regarded me as neutral. Shortly after this, I encountered multiple fleets of this NPR consisting of warhips, which I attacked and destroyed without the NPRs firing a shot. Once the next construction tick occurred, the next NPR fleet I encountered fired at my ships as expected.
It seems to me that the NPR AI is not smart enough to realize that "the other guys are shooting us" should trump "our diplomatic rating was great two days ago" when deciding whether to defend themselves.