What did you see in table T156 - BUSTR ?
This field should have the standard movement type (Even though you create a custom movement type copied from another custom movement type).
My doubt is, your organization has made this field as optional (from MM side) by using different method. I've checked in my system for some movement types, I am not able to see this field as optional (you can see my previous screen shot). So, if I know this movement type (which you have copied from), then I can confirm it.
