C4DtoA 2.6.0 [509cf977]
Arnold core 126.96.36.199
This is potentially a two part problem. The most obvious part is that IPR motion blur consistently doesn't update correctly when the timeline playhead is dragged over all keyframes from a frame within a keyframe range to outside of said keyframe range. (First obviously enable motion blur in render settings and in the IPR). i.e. A cube keyframed on frame 5 at position X: 0 to a keyframe on frame 10 of position X: 1000. Place playhead on frame 6. Click on frame 4. Motion blur will disappear correctly since the cube is not animated on frame 4 and therefore not moving. Now click on frame 6, then frame 5, then frame 4. The motion blur incorrectly remains from frame 6. This problem also occurs respectively for the last keyframes of an animation. This is the first big part of the bug with IPR motion blur.
The second part of this problem which might be solved by solving the bug above is this: With the same animation setup as above, click on frame 7 then frame 4 to reset the motion blur bug to normal. Now click on frame 10 (last keyframe). It shows motion blur on the cube when perhaps it should not. This is a question of whether keyframes should be included in motion blur or not and I'm not sure what the correct answer to that is. Currently you can get either situation where motion blur appears and doesn't appear on keyframes which is inconsistent, so one of those must be incorrect. For certain though is that clicking on frame 11 shows motion blur is maintained which is definitely incorrect since the cube has 0 momentum on that frame.
I hope I explained this well enough. Build the scene with the simple animation if I've made my explanation too confusing.