Silk Touch doesn't always work on Glowstone/Sea Lanterns

What I expected to happen was...:
Every time I mined/dug at a Sea Lantern or piece of Glowstone with a Silk Touch enchanted Pickaxe or Shovel, I expected to receive the block intact.

What actually happened was...:
Around 30-40% of the time (rough estimate) I would actually receive Glowstone Dust or Prismarine Crystals depending on which block I was trying to Silk Touch.
Steps to Reproduce:
1. Obtain Silk Touch enchanted tool
2. Try to Silk Touch a bunch of Glowstone or Sea Lanterns
3. Hope for the best

I had a friend test this in single player (default launcher) as well and he got the same result. That tells me that it isn't something that the server is causing.