It is impossible to scroll a ScrollingFrame horizontally on mobile devices. However, scrolling vertically still works fine. I believe this bug popped up with the most recent ROBLOX update, I started seeing many reports of this in the past day or so.
It is easily reproducable just by creating any ScrollingFrame with a large horizontal CanvasSize and attempting to scroll it horizontally on a mobile device. It doesn’t work whether you drag the inside of the frame or drag the scrollbar itself. I can confirm it is broken on my Samsung Galaxy S3.