I recently incorporated a variable font into a web app at $WORK (as in less then 6 months ago). And I remember seeing some issues like that Using google hosted variable fonts just didn’t seem to work in firefox. There was something about defining the range in the @font-face rule which google font omits (or does differently), or how variable fonts are consumed (font-weight vs font-variation-settings).
My conclusion was—if my memory is accurate—that there was no way we could use google hosted fonts (you should always self host your fonts anyway if you can) and you needed split files for italics (the italic axis simply doesn’t work consistently across browsers).
EDIT: Here is an article describing the current state of the italics axis in browsers: https://rwt.io/typography-tips/getting-bent-current-state-it...
