Busted Heritage Focus?

I put my Stormland (crappy culture) children with a Sand Dornishman (better culture) in my court with the hopes of swapping culture, but it always fails. This is likely a bug, because it works perfectly the other way around (them converting to my crappy culture). The description of Heritage Focus says that it does convert kids to any culture except Valyrian cultures, but I've never seen it succeed, despite sitting a kid with a foreigner for six gosh forsaken years.

Am I doing something wrong? Is the thing bugged? If it's not bugged, why are we forbidden from  educating kids in foreign cultures? Is the Faith mechanism busted in the same way?