Walls get their health bars drawn underneath adjacent wall pieces (this might apply to other objects as well, but it's most noticable with the walls). As this description may or may not make a whole lot of sense, I'll include two images:
First image shows a wall that biters have taken a few bites out of:
Second image shows the same wall, but with an additional row on the next tile:
As you can see, the health bars get obscured by the extra row of walls.
