When an armorstand is holding a slime block in its hand, the slime gets rendered behind objects that it should not be. Chests, Particles, the little black block outline when looking at things in non-adventure mode, etc etc.

Video: https://www.youtube.com/watch?v=P09CR9LFw2M

Command used to summon the ArmorStand:

/summon armor_stand ~ ~ ~ {ArmorItems:[{},{},{},{}],HandItems:[{id:"slime",Count:1b},{}]}