I recently experienced a bug with a favorited text file : when I modified it, the modified version has been saved at the root of my files, not at its location. I ended up with 2 versions of the file : the old one in its correct location, and the modified one at the root. And when I opened the file from the favorites view, it did open the correct (non-modified) one.
I use Nextcloud v2.12.2.5 with iOS 11.3.