It was a bug, but it's now fixed, and the jump controls should be more responsive now too.
Viewing post in Portal Finder Touch Screen comments
Ok cool, the updated version works a lot better the touch controls on the top down levels work better too, but I found that the optimum hand position on those sections was my left hand holding my phone whilst using my right index finger to move and right thumb to jump, it was a little awkward but I didn't mind it. It's a great game!