In certain previously unloaded areas filled with blocks, entities which should be able to be seen through walls are not rendered.

Steps to reproduce:

1. Create a superflat world with a large layer of blocks before the void, such as the "Redstone Ready" preset.
2. Make sure it's been newly loaded. You may need to leave and re-enter it.
3. Face downwards at around 47 degrees and run the following command:
   /summon villager ^ ^ ^15 {Invulnerable:1b,NoGravity:1b,Glowing:1b,CustomName:'"this should be visible"',CustomNameVisible:1b}
4. Despite the summoned entity being both glowing and having an always-visible custom name, it is not able to be seen through the floor. You can go into spectator mode and fly down to it, which will load it and thereby make it visible when you're on top of the floor again.

This behavior does not just apply to newly spawned entities, but also preexisting ones. You can repeat the steps above but instead summon the villager at ~ ~ ~ (current location) and then teleport it down with /tp @e[type=villager,sort=nearest,limit=1] ^ ^ ^15 for the exact same result.