To Reproduce: Place a shulker box, facing upwards, somewhere between y-25 and y-64. Open the shulker box and look through it while it's open (it's easy to notice while the box is in the process of closing). 

It seems like the bottom does render, but renders too low. The bottom of the inside of the shulker box can be seen when there is space below it, like air or a bottom half slab. Stairs demonstrate the problem really well- you can see half of the bottom texture, half of the stairs

I have no idea why it only happens at these y levels, or why it only seems to happen when the shulker box is facing upwards.