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?


I happen to agree with MS, here and that isn't the point.
If you have the doubts you posted, why not either accept what MS suggests, or ask your dictionaries, search engines or thesauruses then come to ELU when they fail you?
– Robbie Goodwin Nov 07 '18 at 23:52