Problem with detecting font features and babel

Question

I'm trying to detect the presence of certain font Open Type font features with fontspec but it fails as soon as a main language is specified with babel.

I have two questions:

1. Am I missing something?
2. Is this actually a big deal? Can I safely apply Kerning=Uppercase even if a font doesn't support it without any weird side effects?

MWE

%! TeX Program = lualatex

\documentclass{article}
\usepackage{fontspec}
%\usepackage{babel} % works
\usepackage[american]{babel} % doesn't work
\babelfont{rm}{Baskervaldx}

\begin{document}
\ExplSyntaxOn
\fontspec_if_feature:nTF {cpsp}
  {Feature ~ detected! \addfontfeature{Kerning = Uppercase}}
  {Feature ~ NOT ~ detected.}
\ExplSyntaxOff

THIS SHOULD USE UPPERCASE KERNING
\end{document}

Answer 1

It’s a bug in the Node rendered, not directly related to babel. It works as expected with the Harfbuzz renderer. If a language is requested and there are no specific features for it, there should be a fallback to the default language.

Sadly, the Harfbuzz renderer is somewhat buggy in hyphenated scripts, so the solution is to set explicitly the language to Default, as suggested by @UlrikeFischer, with:

\babelfont{rm}[Language=Default]{Baskervaldx}

Edit. However, note this is not a general solution. It may work in some cases, but if a fonts defines features for a mixture of languages, with several fallbacks, you are at a loss, because it might be impossible to select all of them for a certain language.