This is a new feature added in NX a couple of releases back. Reset your dialog If you go to menu preferences and user interface, Go to options on the left side of the box, then click on the reset of "don't display this message again".
This fix component only happens on the first component you add. If you add more after that, NX will not ask for this fixed constraint.
I do not know if there is a customer default to keep this from happening.