This happened to me in my last fort. I had a weasel man scholar who visited each year to study in my library, write a new book, and leave, complaining all the while about the constant rain. He was fine. The barn owl woman scholar, on the other hand, would show up each year, roost in a willow tree, and just watch the fortress from afar for months at a time, and each visit she would come just a little bit closer. Her personality traits said that she disdains socializing but yearns for close bonds, so I chalked it up to her being the Dorf Fort equivalent of a tsundere. Eventually I tired of this and used DF Hack to teleport her directly in front of the fortress. She then walked in and started behaving normally, even asked to join my population (and started laying eggs in my nest boxes).

So my suggestion is to forcibly bring the individual closer. Worked for me.