When in midair, you can't begin to sprint when on mobile. This is in contrast to all other platforms/device types where you are able to press a sprint key and begin sprinting in midair. This combined with how easy it is to lose sprint when you get hit makes mobile very bad for pvp (worse than it should be, in any case).

By extension of this you can't sprint while going down stairs unless you are positioned perfectly as when you try to double-tap the game puts u midair from the first tap (you walked off the stairs) and then thinks ur midair for the second tap and won't let you sprint.