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.