Not sure if the method you use does a full game pause but there is some code you want to look at (maybe remove and it works like you expect it). Search for this:
if(ViewFamily.EngineShowFlags.MotionBlur && GRenderingRealtimeClock.GetGamePaused())
{
// so we can keep motion blur in paused mode
// per object motion blur
Scene->MotionBlurInfoData.RestoreForPausedMotionBlur();
// per bone motion blur
GPrevPerBoneMotionBlur.RestoreForPausedMotionBlur();
}
I also suggest to disable motionblur and/or temporalaa to clearly identify which causes the issues. You also can do “show VisualizeMotionBlur” or “vis Velocity”