In my experience, this is caused by a hot pixel on the camera within the small frame used by autofocus. The algorithm appears to look for the brightest point as the star’s centroid and gets fooled by such pixels.
My solution has been to ensure the focus star is not near such pixels when I start (I can see the hot pixels in the image capture).
There may be ‘clever’ ways round this like:
(i) Dynamically subtracting darks, or using cosmetic file list of hot pixels
(ii) Tweaking the algorithm so it ignores single pixels (they surely cant be stars!)
…But I dont know how to access them, or if they even exist.
If not, perhaps a suggestion for next release?