I made a scene with a few lights of which I animated the positions. For some reason, the motion blur makes it look as if it's done backward: the streak of light is not ending at the position where the light currently is in the frame, it's ending at the position of a future frame.
The red line shows the position of the light at that frame, and it's supposed to move down-left in the future frames (frame 35), yet the streak has already gone in that direction on frame 30. How is this possible? This is true for all the lights, on frame 30 they are where they would be on frame 35.