When a sign is clicked from its front, everything works OK. A different story is when you place a sign on a block and standing behind it, clicking its side pixels to edit. Then, an empty "Edit Sign" pops up. Changes seem to be stored, but not applied to a sign. Well, not to its front.
I know it's quite odd to understand due to my lack of English, but please watch a video carefully to understand.

Sincerely.