It used to be that a sidebyside had default margins of zero. This was OK when there was more than one panel, but a bad idea for a single panel.
The new default is the 'auto' attribute value, which will compute a margin equal to half of the (common) inter-panel space. For a single panel, when you specify its width, the result is that the panel is centered.
If you have fine hand-crafted images in your sidebysides (I'm looking at you, Mitch) and were relying on default margins, just add margins="0%" to each case.
If you have been following recent stop-gap advice during the new reliance on sidebyside for one-panel use, you no longer need to add margins="auto". No harm in leaving the existing ones.
Sample article has been brought into sync with reality.
Individual classes of objects going into a sidebyside (eg listing) still need work, but the groundwork is now in place for that.
Rob