Buttons not changing to highlighted state while OnDrag is occuring

I have some buttons under a Panel Group which have a sprite swap animation on highlight. When I begin to drag an item (it's an inventory system) from one button to another, everything works fine, but the buttons do not carry out their highlight function as the mouse passes over them while it's dragging.

The dragged object is immediately parented under a CanvasGroup so it does not block raycasts. Also, if I drag using the Right Mouse Button instead of the left, the buttons highlight normally. However, they don't highlight when I use the Left Mouse Button and drag. However in both cases, OnPointerEnter event is firing correctly.

Why is this happening? Is it a bug?