Urgs.. that's a fun one:

Notice how the underline is all wrong? This happens because the padding of the button isn't 8px as it's supposed to be, but 4px... Looking at the styles:

link_b88.. is the Pivot style which should be applied. However, since the tab is now a CommandButton the CommandButton styles are also brought in: .root_0bd.. .. in our product they seem to be placed in the document in a different order than on the fabric website, and since they have the same specificity, the order in the document matters, and here, unfortunately, CommandButton seems to win.