When Librarians hold up glass, their body where the glass is becomes see-through like the glass texture. I think it might occur on all devices.
To replicate: trade with librarians until one has glass for emeralds. Then go out of trading menu and hold up an emerald. The villager should hold up a glass block. The glass block will reveal what is behind the villager through it.
Linked issues
Comments 2
We're tracking this issue at MCPE-178527, so I will resolve and link this report as a duplicate.
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Edit: Expected result: villager holds up glass block, which just shows the villager clothing through it.
Actual result: Villager holds up glass block, the glass block shows what is behind the villager.