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
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Thank you for your report!
We're actually now tracking this issue at MCPE-48421, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Affects 1.12.0. The only way I've found that you can stack them again is by turning them into blocks and then back into item form.