When editing a 'type in' type of Parameter in Tableau Server / Public the UI from my iPhone the UI looks like this:
Notice that as we are filling up the parameter we see one arrow which UNDO what you are doing. Replacing this button with an OK button will solve the UI conflict because the way it is now an user may type a content in the parameter and hit the arrow to "confirm" and instead it will undo the entry.
Also it is not user friendly that the way you close the parameter control is to tap out of the parameter which is really not obvious at all. Having the OK button or even a X button that closes the parameter is more effective.