I remembered the same bug, but related to the player, when you could fall through a shulker box, which has long been fixed, but armor stands can still do this

Link to video footage where I showed everything in detail: https://youtu.be/5LP4IT2y2PY

To do this bug you need:
1. Place the shulker so that it opens up
2. Measure two blocks up from the shulker and place a block on the third
3. Dig two blocks down under the shulker and place an armor stand on the shulker
4. click on the shulker box and the armor stand will fall down