The fix for this issue is now available in the latest update of InDesign – that is, InDesign 15.0.3.
Please update your InDesign application to get the fix.
Adobe InDesign team
An error occurred while saving the commentScott Lane commented
Well, dynamic spelling still isn't working for me, and I've updated to 15.1.1 (x64). And, yes, I've tried preview mode, character/paragraph styles, and made sure that dynamic spelling is checked and enabled and set to English USA. This really shouldn't be that hard - Microsoft figured this out 20 years ago.