I am having this problem too but strangely enough it was in the earlier lesson. How I fixed that was I was using the regular math node and not the Vector Math node? Could that help you?
Unfortunately, this hasn’t helped here. I am missing something. Once I find, I will certainly check back to this question and see whether this has been resolved or not.
Edit: I noticed I change the Vector Math nodes away from Scale and apply Multiply instead, then it works. Scale is doing something on all vectors and not limiting to Z. Wondering now what does he do that’s different now that Scale was applied to his blend file.
Edit: I got mine working. As odd as it sounds, I just made sure my Vector and Delta Time were in the same receptors of the Vector Math node.