I have never been concerned about subbing yogurt and I have bought buttermilk only once
in the last 35 years. I remove 1 T. of milk from the cup and add 1 T. lemon juice, or lime, depending on what it is.
In fact, if the yogurt has fruit on the bottom, I'll scrape off the top of the yogurt but am not worried about any little bits of fruit that get involved as well.