It's unlikely that it has anything to do with the "calibre6" style, but since you haven't posted the css for this style it's hard to tell you for certain what's going on. It's more likely that ADE doesn't acknowledge a <br> inside an empty <div>. (and the Sony uses the same rendering engine as ADE)
You can use search replace to replace this string:
But then you also need to enable the 'format scene breaks' option under heuristics. I can add this case to the format scene breaks option so it works in the future.