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 commentNeil Jeffries commented
I can't believe this Spellcheck bug has not been fixed. Thousands of us build documents from old version templates – and need to be able to run spellchecks on them. We pay good money for this software, which used to work better, than the "new improved" update. Please revert to the earlier standard and fix this issue on 15.1.2 NOW.