Yeah, Frank was definitely an inward person, if not exactly a loner. Form the time he was rich enough to build his home studio, he would spend every waking hour in it working on composing and arranging music, which was the only entity outside himself he had any passion for, to the detriment of all the other people in his life. He probably had some diagnosable condition leading him to withdraw, but he was also intelligent and insightful enough to seek help for that if it ever bothered him, which to all appearances it didn’t, since it led to his fame and fortune.
There was a very good Washington Post article from a couple years ago about how his kids are divided from one another, and Frank does not come off looking good.
https://www.washingtonpost.com/entertainment/music/interactive/2024/frank-zappa-family-feud-moon-unit-dweezil/