Confirmed on Apple iPad mini.
This happens to repeaters and comparators also.
Also, this is a duplicate of MCPE-16225.
This works as intended. When a block is being pushed by a piston, it turns into an entity, rather than a block, so that it can be animated. So while the piston is in between states of extended and retracted, it turns into an entity and some physics do not apply.
Confirmed on Apple iPad mini.
Confirmed on iPad mini iOS 9.3.3.
Are you sure you hadn't provoked them another time you were in the Nether? Also, was this in Survival mode?
Never mind, for some reason, when I was exporting my skin from my MacBook to my iPad, the transparency didn't keep. By going to this website on my iPad and saving the image directly onto it, my skin did not become corrupted, and worked.
In short, this was nothing to do with Minecraft. It can be resolved now.
Hmm... strange. What do you mean by 'black transparent'?
Sorry, i meant 0.15.2