When generating a world, newly generated chunks are generated with a fixed light level of zero (likely due to skylighting not yet being calculated).

This causes any worldgen features (such as plants) that are dependent on a non-zero light level to fail to generate.

This will cause plants like mushrooms to generate incorrectly, as they will always determine the light level is 0 during worldgen, when they may otherwise be exposed to sunlight.

This is also inconsistent with itself. If you teleport to a given chunk, the light level will be calculated prior to worldgen, causing the feature to generate as normal, however the surrounding chunks (and any other chunks you fly/walk to) will fall back to zero.

It can be reproduced by putting a simple placed feature of type "simple_block" that places a feature every chunk, then test out flying vs teleporting to new chunks.

I tested this specifically with the vegetal decorations generation step