I bumped to this issue many times in my texts. I like references. In my texts, I add a lot of references: to chapters, to Wikipedia articles, to glossary at the end of a document, to 3rd-party sites, etc. After all, I am writing for www, and links is a core feature of web. Most of my links are “shortcut reference links”, If one shortcut reference links follows another ([a] [b]), markdown processor (I usually use pandoc) interprets it as one reference link with text a and label b.
It is very confusing. I have submitted a bug, but jgm said it is a feature — it is the way the original markdown works.
However, I do not see rationale for such interpretation. To me it looks inconsistent.
I understand jgm argument about backward compatibility, but I believe allowing a space between link text and label is a mistake. It hardly possible to fix it in original markdown, but it can be fixed in CommonMark.