Enforced supports are internally considered as very stiff springs by STAAD.Pro. So when you specify it as enforced, there are additional active dofs for the first analysis compared to the second one with FIXED support, which is making it work.
You may refer to the following documentation for additional details on Enforced Support
Section 5.27.1 of the Technical Reference manual titled Global Support Specification
You may also go to Help > Contents > Graphical Interface Help > Modeling Mode > The Menus in the Modeling Mode >Commands Menu > Support Specifications > Create Support dialog > Enforced tab for some more information