Signs that were placed with a setblock command that doesn't specify the content of all lines are converted to completly blank signs when the world is reloaded. If you use pick block to get that type of sign, the item count goes negative when you place it. However when it's placed, it places a regular sign.

/setblock ~ ~1 ~ minecraft:standing_sign 0 replace {Text1:"{\"text\":\"This is a sign\"}"}

Also occurs with the fill command. Does not occur with blockdata.