1.11 grindstone turns into item

I placed a grindstone on a Wall with the Wooden Parts on both sides of the Grindstone facing to the wall, after a certain amount of the time playing in the world, the Grindstone became an Item that layed on the ground. 

I then placed the grindstone with the wooden parts facing on the ground, and it seems like that the bug doesn’t occur anymore. 

Solution: because I wasn’t in the chunk where the grindstone also is in the time where the grindstone became an Item, the Chunk made it turn into an Item. 

But this is just an assumption