When a block is moved by a piston, it is replaced by a moving block in order to be properly animated. However, that replacement doesn't occur instantly, it creates a brief overlap when the piston extension/retraction starts. This can be observed more easily with semi transparent textures (such as stained glass, ice, slime blocks or honey blocks), as their overlap will affect the texture's opacity.
Screenshots/Videos attached: Yes
Notes: In certain occasions, MCPE-136928 may occur as well, making the effect easily observable with any block.
Linked issues
is duplicated by 4
MCPE-174367
Blocks appear to render twice while being pushed by piston.
Resolved
MCPE-180137
Piston pushing glass block behind other glass block not properly culled (possible duplicate)
Resolved
MCPE-184639
Slime block and honey block
Resolved
MCPE-186570
Moving transparent blocks were rendered before other blocks regardless of their positions
Resolved
Attaching another video example (game speed was slowed down by external means only to show it easier; ignore the water; was for another bug report)