I am used to spelling "naïve" thus - "naïve". I am also used to Microsoft Word automatically changing "naive" to "naïve". Hence, I was surprised when it didn't change "naivety" to "naïvety". I then decided to work around this by letting it correct "naive" to "naïve" and then tacking on "-ty". Word then underlined "naïvety" in red and suggested a correction to "naivety".
Whether this is a typical case of "it's just a quirk of spellcheck" I am not sure, hence my question.
So:
Is it incorrect to write "naïvety"?
Alternatively:
Why is "naïvety" incorrect when "naïve" is correct?