Breaking "grass" has chance to drop "Double Tallgrass" (purple/black block for seed from it)

Breaking grass has glitch image of sunflower briefly sometimes, grass drops DOUBLE TALLGRASS block sometimes, that can be replaced and broken multiple times until a seed drops from it, THEN the tall grass does NOT drop again ONLY the seed. The seed that drops eventually from this DOUBLE TALLGRASS block appears as a small black/purple cube, but DOES stack with regular seeds when picked up.

What I expected to happen was...:
When breaking grass, i expect to have a chance for a seed to drop. As of right now in 14w26c there is also a chance that grass can drop DOUBLE TALLGRASS Block which can be placed on dirt, and broken and picked up multiple times until the seed drops.

What actually happened was...:
When placing theDOUBLE TALLGRASS that is gathered from breaking grass, the tall grass has a small chance to drop a seed, which the seed looks like a black/purple cube, but DOES stack with regular seeds.

Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... Break grass until a "Double Tallgrass" block drops.
3. ... Place tall grass block on the dirt.
4. ... KEEP breaking the tall grass block and placing it until the glitched seed drops.