that's a good question!
It seems that QV automatically saves the space for the expression header if only one expression / expression group is used. No header, no button!?
If you add another expression, you will get the header with button, but that's probably not what you want.
Haven't found a way to get the header back, but maybe I am missing something.
Seems like a bug to me (or too much automatism applied), but QT probably would say, works as designed.