Whenever a player uses the Middle Click (default is the scroll wheel) onto a sign, or creates a sign using commands/Command Block, you should notice it will say as normal "(+NBT)" underneath the name of the sign. However if a player places this sign, it'll act as if the sign has no NBT, and work as if it's a normal sign.

This bug so far only happens for signs, for example a Chest with a NBT in it will still contain the NBT.