Custom blocks do not work, and crash the game when they are referenced. I've seen crashes in the following scenarios:

1. Custom block in addon. In console, type "/give @s "
   1. Game crashes when the space after "@s" is type
2. Custom blocks are used in world generation using feature/feature rules
   1. Game crashes during new world generation
3. When a custom block has "register_to_creative_menu" set to "true"
   1. Game crashes on world load

My custom block addons worked fine a couple of weeks ago, and I have not changed the code since then.

Attached is an addon containing a simple custom block to demonstrate scenario #1 from the above list. This block uses standard "stone" textures that work with vanilla blocks, so it seems unlikely that it's a texture issue.