The bug
Slime blocks and honey blocks appear to be fully transparent while being moved with a piston. The smaller inner cube is missing while the block is moved.
MC-163404
Inner parts of slime blocks and honey blocks disappear while the block is moved by a piston
Resolved
Fixed
7
Can confirm, affects slime blocks as well. Unsure whether this might be intended due to a technical limitation, however I couldn't find an older ticket for this.