The builtin fontspec.cwl is out of date -- https://sourceforge.net/p/texstudio/hg/ci/default/tree/completion/fontspec.cwl says v1.14, July 2007. However, fontspec is now on v2.7: https://ctan.org/pkg/fontspec
As a result, several commands are not recognised, e.g. \renewfontfamily
Anonymous
developments has moved to github (https://github.com/texstudio-org/texstudio)
You can provide us with an updated version of fontspec.cwl ... (on github)