I turned off the auto update when I noticed stuff happen with 2 books I was not happy with. Those are just the ones I noticed. One book was basically a full book when I got it, then it was updated and turned into a chopped part of a book as the author decided to chop it up in 4 parts. So they basically took back what I had already and gave me a sample. I didn't pay for that book.
The other book I did pay for and the author decided to do a PC cleaning on a back list title book. It was from the late 80's or early 90's I believe, I read it and then found out the update took out scenes and changed the tone of the book. There is a reason I love the back list stuff. Thankfully, I had already "liberated" that book in calibre so I'll always have the true unbutchered version. But all this made me very concerned about what else I missed, what other books were mangled and I hadn't noticed yet.
Now I decide on a case by case, but no auto for me. I actually don't know anymore when there is an update, I don't get the emails anymore and they removed the drop down they used to have for updates. So I read the books the way I acquired them. If errors need to be noted in a review, I will do so. I guess I expect the version I buy to be as good as it can be, just like when I buy a paperback. I only go buy the version I have.
So I don't notice anymore.