I was saying "must have went" is perfectly normal in many parts of England, where I am from its the default way of making that expression.

Sorry I do dislike language elitists, the English Language varies massively around the world, its great actually, after 15 years as a seaman, working around the world, mostly around the English speaking countries, it is fascinating how differant our language can be.

Except one country, that seems to lack the ability or will to communicate with others, that laughs at unfamiliar expressions.

My first experience in the USA as a child I was asked how I learned English, after telling the person I was from England lol.

I dont believe Americans are stupid, insular perhaps, with friends, family and my own working experience scattered around UK, Australia, Canada, New Zealand, Falkland islands, we all understand each other fine and get on, only yanks feel the need to make a big joke about a phrase or word that isnt normal to them, it is tedious, and tiresome, and does not reflect well on your country.