Coal mined with silk touch pickaxe don't and up with other coal

Coal don't add up. 

 

Edit: today, with the new update i still found a stack of coal that did not want to add up with another stack of coal. I noticed that i mined the coal with a enchanted pickaxe with silk touch, so i put it in the oven to get the coal. That one did not add up with other stacks of coal. 

 

see video