Manually created order - can't edit product price

[30knees]

I'm manually creating an order in the back-end. 

I can't edit the price for a product that has a quantity of 2 or more; the price always jumps back. Only when the quantity is 1 does the manually edited price stay. 

Oddly, this only affects a certain (attribute) product. Other products aren't affected. 

Any ideas what this could relate to?

Edited May 20, 2019 by 30knees