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 commentAlison commented
I am having exactly the same experience too. Spellcheck refuses to work with new text in an old document, but does work if it's a brand-new document... and it spellchecks some older documents, but bizarrely not others... all very confusing. Very much looking forward to this bug being resolved!