I've encountered this problem in (I think it was) 2021.2.0a13, and it is still present in 0a16. The issue is that scriptable object assets are not selectable/viewable in Asset Select window (opened, for instance, from object reference field within inspector), while I am still able to drag & drop said assets into the field from the project files, so it's not a coding error. I tried to create an empty project with one scriptable object class and asset instance, and one monobehavior with object field of SO type. The Asset Select window doesn't show anything, but the drag and drop assignment works. From which I assume it's most likely a bug.