When a variable font has one file for Roman and another one for Italic, the VF sliders only work if the STAT.AxisValue is in format 2.

STAT.AxisValue format 1 or 3 works in Illustrator 2020 and also in Photoshop 2020 if the font has a nameID 25 and postscriptNameID attributes for fvar.NamedInstance (a bit weird since Illustrator is fine without that).

If the files have only nameID 25 and postscriptNameID but the various STAT.AxisValue aren't in format 2, the named instances work in InDesign, but not the sliders: you can't choose an arbitrary value reliably.

Adobe obviously likes to make VF with STAT.AxisValue in format 2, but it would be nice if InDesign works well with fonts from other vendors, done differently. For example, since the end of June, new VF releases at Google Fonts have nameID 25 and postscriptNameID, and good STAT table (but no AxisValue in format 2). Could you check some fonts, like Bitter[wght].ttf and Bitter-Italic[wght].ttf https://github.com/google/fonts/tree/master/ofl/bitter and make sure they work well in InDesign (they already work well in Illustrator and Photoshop)?