When I click “wear hat” it shows that one only. I can only wear one at a time. The position controls are applying to all hats, not just the selected one, too.
This is not a bug.
That is how it is supposed to be, and that's also how it always was.
If you really desperately want to wear multiple hats, crop the hats using some sort of screen cropping tool, apply them to your avatar using an image editing tool (i.e. Paint), and replace your current avatar with the modified one.