As can be seen from the screenshot the GUI for writing on the sign is both clipped on the right side and too tall (as shown by the 1, 2, 3, 4).

You can see it's not an issue with the actual entity texture as you can see it looks correct in the background.

I see no way to correct this manually in the resourcepack via any of the numerous UI json files.

EDIT: You can also see by the 1, 2, 3, 4 in the entity the text isn't vertically aligned centrally on there either.