In my world, 'alternate history' covers the sub-genres 'alternate history,' 'historical fiction,' 'historical romance,' 'historical fantasy' (what I write sometimes) and almost anything else set in the past. They aren't the past as it was lived - they are the past as it is novelised.
Fictional pasts are things I love and I will recommend lots of wonderful writers to anyone who asks. However good they are, they are *not* the same as history-as-academic-nonfiction because the story-telling component has to be more important than the communication of the technicalities of past human experience. They're all the better for expanding and changing the past and adding information we can't possibly know in order to give a joyous reading experience. They're way more readable for not using the precise mindsets of a given period, because we can understand a world more comfortably when mindsets in that world are closer to our own. They're not to be trusted as your chief source of historical understanding. They're fiction.
Is that clear enough? Or do I need to sulk?
Yes, I got into another "But I read it in my favourite novel so it must be true" argument. I need to develop a thicker skin.
Some writers who take good care with the past and will give you most enjoyment in novels set in the Middle Ages include Elizabeth Chadwick, Brian Wainwright [though not his Alianore Audley book, which is a joyous romp but intentionally anachronistic], Felicity Pulman, Sharon Kay Penman, Candace Robb and Sharan Newman.
The writers who take the most care with the Middle Ages and don't write fiction number at least 4,500 internationally and I'm not even going to start to list them. I can give you favourite writers on specific subjects if you ask nicely eg Judith Bennett on women's history or James Brundage on canon law and how it treats sexuality.
Rant over. You can stop laughing now.