The bug
Transparent blocks, such as stained glass, slime blocks, or honey blocks, as well as blocks with special renderers, such as chests or signs, do not render behind transparent blocks that are moved by a piston.
Video
[media]The platform of moving blocks that was used in the video is attached to this ticket.
Linked issues
relates to 2
Attachments
Comments 6
relates to MC-164131
They don't render behind transparent blocks in general, see MC-161899