I find it hard to see how you might have come to that conclusion. If you perform an internet search for "Kirby Muxloe Interchange" there are numerous results pointing to J21A, and using the name word-for-word. 

Whether a name is used in an official capacity or not, that doesn't detract from its existence in common usage. As I'm sure you're aware the name=* key is intended for the name most commonly used. If not for the junction number being quicker to spell out, this would be the obvious way to refer to the junction.