As far as I know, the lwc auto locks that block when you place a sign, this is inorder to prevent the destruction of the block that the button/lever/sign is on. if you do destroy that block(when you are the owner) there's no reason why the sign/lever/button wouldn't be unlocked aswell.

if you are refering to an other block which the button/lever/sign was not placed on, that might be a bug.