There's been similar accidents like it
not recognising an overturned truck and ploughing straight into it. The vision system they're using seems to be, at least partly, using object recognition to spot other vehicles so it's been trained on millions of photos of road things. The problem with that is when it sees an object that's not sufficiently similar to what it's been trained on, it doesn't go "WTF is that?", it just doesn't recognise that there's anything there at all. Presumably the dazzling flashing lights prevent the vision system from recognising that the cop car is a cop car.
At least it's got out of its habit of following joints in the road surface instead of the road markings.