I’m sorry to tell you that as far as I am aware is a physx issue that has persisted since UE4 came out where time dilation doesn’t work correctly with physics. I think its been reported but I’ve never seen a fix log for it. Physics runs on a seperate thread from the game thread and since it doesn’t appear to be taking up the dilation at the same time it likely would have issues since it would be running the simulation at a different speed from the game thread, or at least that is what I assume from the jumpy behavior I have seen it cause.
I wouldn’t know where to look for a fix, never had a need for slow motion physics so I have never looked at it.