Killing a mob with weaving effect sometimes only spawn cobwebs half the time

When killing a mob with the weaving effect on, it sometimes only spawn cobwebs half the time.

Steps to Reproduce

1. Create a default Flat world
2. Spawn a mob (smaller ones such as chickens)
3. Throw a splash potion of weaving on it
4. Kill the mob
5. Repeat the process

Expected Result
The cobweb should always spawn

Observed Result
The cobweb doesn't always spawn when you kill the mob.