When typing in a interface like a sign or a book the cursor will position itself one space ahead of the current typing location for every continuous line typed (not pressing enter).

For example: in a sign or book

Line 1: The cursor is where it should be: TEXT|

Line 2 without pressing enter: the cursor is one step ahead and you can't see it: TEXT |

Line 3 without pressing enter: TEXT  |
And if you use the arrow keys to move the cursor to where it should be it will type the number of continuous lines characters behind it.

Please watch the video below if you didn't understand this.