R.J.
The mirena doesn't shorten or lessen the flow of ALL women, just most women. It's hormone based, and most hormone based birth control shortens periods (hence why people get heavier flows when they have the copper iud, use another non-hormone method, or are on no form of birth control at all).
But no matter what form is used SOME women have the "opposite" reaction. Like most women on depo lose their period entirely, but some don't stop bleeding for the entire 3-6 month time that the shot is active. MOST women's skin clears up on hormone BC, but SOME break out like they're teenagers. Etc..
Having 2 week periods for 6 months doesn't sound like an adjustment period. It sounds like you're an "outlier". AKA are responding atypically, but certainly not uniquely.