I have found a bug with motion blur and photomode. With motion blur disabled in the graphics settings and entering photomode and enabling motion blur from the photo effects the car is a blurry mess, regardless of intensity. From what I have tested, if motion blur is disabled in the graphics settings it will not work correctly in photomode, instead blurring the car.
In order to resolve it, enable motion blur from the graphics settings, unpause and pause the game again. Motion blur in photo effect will now work correctly.
Also, if motion blur is already enabled and you disable it from the graphics settings will not break motion blur in photo effects if you haven't unpaused the game in the mean time. So it's a conflict between graphics motion blur and photo effect motion blur when it's disabled graphically.