@macdork Are you sure that this is an Editorial feature? I'm no expert on Editorial, but on my iPhone nothing special happens when I type multiple underscores, and I didn't see any setting to enable that sort of conversion. You might want to check if you have an Editorial snippet or an iOS text replacement (Settings app > General > Keyboard) configured that replaces two consecutive dashes with an em dash.
Guess what? It was the OS-level Smart Punctuation feature. I suppose it's on by default, but turning it off fixed my issue (in iOS 11 public beta).