I've noticed that in some modal style screens on the Wikipedia mobile web that dragging/flicking doesn't reliably result in scrolling. Usually, if I try, say, 5+ times, it eventually activates scrolling, though.
Two places where I've observed this are as follows:
(1) Tapping the edit pencil. I tapped the edit pencil on the lead section of "Joe Beef (restraurant)" while not logged in on the stable channel. I see the anon editing warning, and the first 2.5 buttons below it, but regular dragging/flicking is spotty.
(2) Attempting to scroll the results list in search in alpha / experimental mode. When I was getting a list of results I think from the auto fulltext mode last week or the week before, although there were obviously more results than could fill the screen, dragging the list didn't do anything. It seems at least this morning that it's in prefix mode on alpha / experimental, so I'm unable to reproduce.
The edit issue I've only seen on iOS 8.3, because I've only tried anonymous editing on 8.3.
The alpha / experimental symptom was present on iOS 8.2 (8.2, not sure about 8.3 - just upgraded!) on the 5c device as well. Unsure if it's related to Private mode browsing at all, but wanted to clarify in case there's something strange with LocalStorage or what have you.