The "handwriting" data for this model is basically the coordinates of a pen. The length of the string representation of the text is very different from the length of the coordinate representation of the text, therefore the model "learns" a window corresponding to when it is drawing the current letter, and when to start the next letter. For these letters, as the model doesn't learn how long this window should be, nor how to transition from it to the next letter, it gets stuck and outputs nonsense.