Are you sure that you are not hitting the undo key on the keyboard or accidentally selecting the text and deleting it? I know that sounds far fetched but I have seen crazier things happen.
If you are sure that you are not causing this to occur, reboot your iPad and see if that makes this issue disappear.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.