Proper nouns are words. That answers the prompt, right? There was no mention of "common" in the prompt. If there was, the list of words I got with the same prompt on ChatGPT would have been a lot shorter.
Then the word rhamanagagi (which I just made up) is a word that would technically belong to the list just fine, it definitely not answered to the implicit intent of the question.
The strength of LLM is their ability to answer to unprecisely specified questions, being able to guess the speaker's intent, but in this particular case, it's failing the test.
https://www.google.com/search?q=stagi
There are a lot of genuine hits for stagi.