I found the following checkin after digging through the multiple branch histories:
It appears that SimpleFold has always performed this rather odd conversion, but I am unable to find any reasoning behind why it is considered valid.
ASCII characters should not convert to non-ASCII characters. According to the W3C, https://www.w3.org/International/wiki/Case_folding, folding for the purposes of comparison should be a two step process. Normalize, and then case. SimpleFold appears to attempt to shortcut that, but based on my understanding of the Unicode spec, doesn’t do so correctly. I believe this to be a bug, and unless someone can explain what I am overlooking, or similar functionality in another language, I will be reporting it as such.