When editing sign text, pressing any arrow key (even the left/right keys, which are supposed to do nothing) inserts (what seems to be) a blank character.
To reproduce, place a sign and type something. Now press any arrow key and try to delete the text. The blank character will get in the way. Next try entering some text, pressing an arrow key a few times and typing something else on that line. The game will allow you to do so no matter how many times you press an arrow key, because signs now check for character width. Now try to delete that text. The second half of what you typed will delete as normal, but when you get to the point at which the arrow keys were pressed you will have to press backspace the same amount of times you pressed the arrows. This leads me to believe that the game is indeed inserting blank characters.
I would provide screenshots, but they wouldn't be very helpful. I don't think a video would be of much use either, though I can provide one if need be.