The bug
Shulker boxes appear to be a couple of screen pixels shorter than other blocks, which lets animated textures or particles (like torches) appear through a small gap between the blocks.
How to reproduce
Place a shulker box
Place glowstone behind the shulker box, and surround it with blocks
Use a spyglass to look at the edge of the shulker box
❌ A few screen pixels of the glowstone are visible
Hi there!
I can confirm in snapshot 20w29a and release 1.16.1.