The bug

Falling blocks are not rendered if the block state they are displaying is at the block position of the entity. The shadow of the falling block is still rendered.

Expected behavior

The expected behavior is that falling block entities are always rendered. Otherwise the player might be confused why he sees a shadow but no entity or why he cannot click / hit at a position because he cannot see the falling block entity blocking his view.

Even with regular falling blocks which begin to fall changing it to have the entity always be rendered does not make a visible difference.

How to reproduce

1. Place a block, for example a trapdoor

   /setblock ~2 ~ ~ oak_trapdoor
   

2. Summon a falling block entity displaying the block

   /summon falling_block ~2 ~0.9 ~ {NoGravity:1b,Time:-2147483648,BlockState:{Name:"oak_trapdoor"}}
   

   Note: The block will probably get invisible because of MC-72248. Right clicking the trapdoor block should solve this.
   → You should see that the falling block entity is not rendered unless you break the trapdoor or change its state, for example by right clicking it

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.client.renderer.entity.RenderFallingBlock.doRender(EntityFallingBlock, double, double, double, float, float) does not render the falling block entity if the block state it is displaying is at the block position of the entity.