Setting a field or other object as a button turns it into a group. In FM16 you have to use the new Layout Objects palette to select the field within the group.
I agree that it's weird that it remains a group even after you remove the button attribute. The fix is to un-group it, but you should still report this as a bug.
I'm not sure if this is technically bug. It is definitely a change in behavior. As soon as I select it in Layout Objects Pallet all the data attributes light up!
Just have to get used to the new feature.