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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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?
WAI. MC-47859