Reverse sign is not working when flowfield is based on another flowfield

Hi everyone,

I created a flowfield FieldXX1 with the following calcformula: -Sum(tabXXX.Quantity where fieldXX2=Yes);
fieldXX2 is another flowfield.
The formula gives the correct value but the reverse sign is not working: instead of 10 I get -10.
Why is that?

Thanks in advance