So your 3D boxes are likely being effected by your motion blur coming from your post process volume. That’s unavoidable unless you were to either remove the motion blur or you could try to use custom render depth and using that to exclude them directly from your post processing. Got a comment here from a relevant thread with some solid resources. It’s a bit older but still checks out. Though I don’t have a direct tutorial for disallowing post processing on specific objects, this would be the path to go.
How can i exclude specific assets/blueprints from post process? - #7 by ryanjon2040