Breaking(-ish)
MDX just hit 1.0 and one of the changes they made is changing an MDX document/component from being wrapped in a <div> to a <React.Fragment>. Using a Fragment makes sense and is more in line with what people probably expect.
For most people this change isn’t important and they won’t notice a difference in their sites. I tested it out with a few of my sites and they all looked the same. However, the CSS on one of my sites specifically targeted the <div> wrapping the MDX document and when that <div> vanished it visually broke the site. So you should test out each site after upgrading to make sure it looks right.