The comparator is missing it's bottom texture. It should've been given one in 1.8 like all other blocks missing a bottom texture were. The barrier block makes it possible to see the bottom in vanilla.
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WAI. MC-47859

Incorrect. A bunch of blocks were given bottom textures in late 1.8 precisely because the barrier now exposes the bottom. The intended behavior is that all blocks have a bottom. That old bug report was from before Mojang made their intentions regarding this clear. Please reopen.
EDIT: Ladder was one example, although that's a back texture not a bottom texture.

Incorrect. A bunch of blocks were given bottom textures in late 1.8 precisely because the barrier now exposes the bottom.
Source?

I looked on the wiki after posting that and Ladder was the only example I could find. Maybe I read too much into that 1 change.
EDIT: oh the 4 pressure plates have bottoms, although I'm not sure they were missing them in older versions.