Caused by rBac442da4a14d.
Above commit tweaked the logic to not only early out, but also set the
WPGradient_vertStore screen coord to FLT_MAX in case this original index
was visited before [gradientVertInitmapFunc].
For generative modifiers though, we might get here multiple times for the
same orig index, resulting in a valid orig index being made invalid for
gradientVertUpdatemapFunc [which would early out in case of FLT_MAX].
Restored original logic, so that setting FLT_MAX only really happens
when it should: when ED_view3d_project_float_object fails...