"twelvemonths" recommended over "twelve months"

What got flagged?

Over the last twelve months, ...

upon which, Harper says

Did you mean the closed compound noun "twelvemonths"?

Why is this incorrect?

Well, apparently "twelvemonth" technically is a word according to dictionary.com / Collins, but I can't say I've ever come across it (my browser-native spellcheck as I write this issue is also of the opinion that it's not a real word). Looks to be some weird archaic term that nobody actually uses any more.

Either way, I don't think it's something Harper should recommend over the perfectly-grammatical "twelve months"