Basically if you have an armor stand wearing or holding a translucent item model (e.g. stained glass), which can be summoned using the following command:

/summon minecraft:armor_stand ~ ~ ~ {ArmorItems:[{},{},{},{id:"minecraft:orange_stained_glass",Count:1}]}

and look through the translucent parts, the following problems occur:

• Translucent blocks (namely, other stained glass blocks placed in the world) do not render at all through translucent item models.
• Some entities also will not render. It seems to be purely random which entities do or do not render behind a translucent item model, it does not seem to depend on whether the entity was spawned before or after the translucent item model was summoned, or on which direction through the model you look in. Furthermore, if you have two different armor stands wearing translucent item models, it is possible for a particular entity to be visible looking through one of them but not visible looking through the other. (Fixed in 19w42a)