I can't find the idea where this was originally discussed, but I do remember after button category was introduced there was a discussion about behaviour when there was only one button left in a category.
It had originally been implemented as showing the Category as the button label.
After much discussion, and I was a strong proponent of this alternative approach, it was agreed that the button label would be that of the remaining button, not the category.
I now have to admit that I may have been wrong as I find an increasing number of circumstances where I have buttons with mutually exclusive filters within a category and I always want the Category name displayed, never want the Button name displayed, unless of course by some chance the filters were not mutually exclusive.
I see merit in both approaches and wonder whether each button categories could be given a life of their own in which we can define by category the behaviour when there is only one displayable button.