Browser Spell check is not working for new custom fields
spell_check field on dictionary record is not checked
-- Navigate to System Definition --> Dictionary
-- Search for the field on which spell check is not working
-- Check Spell Check checkbox
Note: This field is not on the form OOB you will need to configure form layout to add the field
-- Save the Dictionary record
UI15 uses *only* the browser spell check, foregoing the legacy ServiceNow spellcheck capability, so the Dictionary field is used for that.