This issue has been present since 14w31c and (possibly earlier), but does not occur in 1.8.8.

Any cobblestone wall (or mossy variant) will incorrectly detect a block above it ("up: true") except where west+east OR north+south are actually true.

Only N/S or E/W facing segments behave correctly. All other segments, including free-standing "post" segments detect a non-existent block above.

The adverse effect is noticeable in any resource pack which uses the "up" attribute for a specific model.