When you reload a world with NBT signs, the signs will be blank.
This only happens with the signs using JSON formatting.
Normal signs placed with hands are not cleared.

This is the command I used;
setblock ~1 ~1 ~-1 wall_sign 2 destroy {Text2:"

{\"text\":\"Rainbow Mode\",\"bold\":true,\"color\":\"dark_blue\"}

",Text3:"

{\"text\":\"The SpellBook\",\"color\":\"dark_gray\"}

"}