Hi Kattie,
this is a new bug with InDesign 15.1.2.226 and some instances with the usage of variable fonts.
You may find a bug discription at InDesign UserVoice.
Ah. After a quick search:
Underline with variable font
When the underline of a variable font is activated, the underline takes the whole height of the page. (Indesign 15.1.2)
François, August 31, 2020
https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/41275063-underline-with-variable-font
My advice: Go back to the previous version of InDesign.
You can do this by uninstalling and re-installing through the Creative Cloud Desktop application.
Regards,
Uwe Laubender
( ACP )