Leaving a world with deferred rendering enabled won't render blocks in thumbnail

Steps to Reproduce:

1. Import any resource pack with PBR capabilities;
2. Enable the experimental toggle for Render Dragon features;
3. Switch rendering mode to "Deferred Technical Preview";
4. Leave the world.

Observed Results:
The world's thumbnail is a blank skybox.

Expected Results:
The world's thumbnail renders the blocks.

Notes: This wasn't an issue before this version.