Codespan currently highlights in red whatever the primary label is pointing to in the source, while this looks great for errors inside of source code lines, it looks really ugly if the label points to the whole line. It ruins the point of highlighting, you can clearly see what the label is pointing at, a line of all red just looks like it does not belong there. In the case of multiline labels, usually they are used to point to blocks of source code, but this means if you highlight everything inside of them you end up with a giant blob of RED which just looks like it does not belong and it ruins the cleanliness of the diagnostic.

Labels should highlight their source code only if the label points to a subsection of the source code line, and should be fully disabled for multiline labels.