This is what appears to be an incorrect block outline for potatoes (may affect other plants too) and it is for the age 3 of the plant. It's supposed to go to the bottom (i'm assuming) but does not. It is also the same size as age 2 outline of the plant, but a little bit above the ground. It's specifically the outline of the block, since i remember prior to this update ages 2 and 3 were the same regarding looks, but the block outline sizes differed.