It could be a glitch in the variable font file that InDesign doesn't like. If the version of Mulish at Adobe Fonts isn't working correctly you could try downloading the version at Google Fonts to see if it works better: https://fonts.google.com/specimen/Mulish The ZIP package at Google Fonts includes a couple of variable fonts and a sub-folder containing 16 static fonts.
Muli is another alternative. Mulish is a variant of Muli (both designed by Vernon Adams). Muli isn't available at Google Fonts, but it can be downloaded at Font Squirrel: https://www.fontsquirrel.com/fonts/muli