(I am in version 0.13.1 but the drop down menu will not show it as an option) I made a lava blade for my iron golem farm. Because we do not have hoppers I need to be able to add the lava and take it away. When I put the lava down the signs did not burn but when I took it away the signs lit on fire and burned up. Further research showed that it had to be a 4x4 or bigger hole and it did not matter if the signs were on the wall or the ground.
Steps to replicate
1) make a hole at least four by four and place signs in it
2) put lava over the signs
3) take away the lava
4) watch the signs burn