base mode ignores character variants if math script style is activated

[marcin-serwin]

MWE

\documentclass{article}
\usepackage{fontspec}

\begin{document}
\fontspec{STIXTwoMath-Regular.otf}[
    Renderer=Base,
    Script=Math,
    CharacterVariant={3},
] ∅ % Correct
\fontspec{STIXTwoMath-Regular.otf}[
    Renderer=Node,
    Script=Math,
    Style=MathScript,
    CharacterVariant={3},
] ∅ % Correct
\fontspec{STIXTwoMath-Regular.otf}[
    Renderer=Base,
    Script=Math,
    Style=MathScript,
    CharacterVariant={3},
] ∅ % Incorrect
\end{document}

produces

All three symbols should be identical.

Causes latex3/unicode-math#587.