If lines or words seem to be disappearing, please check if the 'Remember and Repeat' option has been accidentally switched on in the settings menu.
What is Remember and Repeat?
When switched on, Remember and Repeat replicates the standard classroom spelling strategy Read Cover Write Check. A student types a word, it disappears and they immediately type it again from memory, benefiting from the extra repetition.
It is recommended that this feature is used when a student reaches words that they find particularly difficult to spell.