The problem with reading mode is that sometimes it doesn't display everything on the page, such as the comments section. This bug pretty much renders Edge useless for trying to read anything on your phone, and proves to me that no one at Microsoft actually uses WM on a regular basis. This bug has been around forever, and no one with any say-so within that organization would tolerate it if they actually used it.
I agree that it seems to be the constantly updating ads that cause it.