Behind the painting, the plank texture is outdated with pixels from the Classic Textures resource pack. To see this, you must give yourself a barrier block, place the painting on the barrier block and go behind the painting.
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Yea, I reported this same thing, but with a file attached with a new texture to it that I made.
Reopening as MC-160388 was reopened.
This isn't a bug, Jasper just didn't change that texture.