Registration area for touch moves sometimes!

I’ve got a weird bug that is killing my app on iOS. Sometimes (and i can’t find a pattern) when you hide the keyboard after you have inputted text into a form or modal, it moves the registration area of your touch!

to try and explain:

You add your text in the input field. The keyboard hides again, and If the username is taken then you have to try again. Only the area you click to input the text has now moved above the input field, so clicking on it makes it seem like it doesn’t do anything, and it seems to have frozen.

i’ve included a graphic to show it better than i have explained it (hopefully). If anyone can help, it’s killing the app as people aren’t able to click on the input again and think it’s broken!