"Add a Custom Summary" is at the bottom of the form, and collapsed by default. That's fine if it is not used, but less fine if it /is/ used. It becomes especially problematic if a translation module such as Fluent is used, because then you'll happily translate the Title and Content, but forget about the Custom Summary.
After all, it says "Add a Custom Summary", so there's no visual indication whatsoever that something is filled in there.
My suggestion is to simply uncollapse it (onload) if it has any content.