coal and coal ore that went in a furnace won't stack
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Thank you for your report!
We're actually already 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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This is because any object cooked/smelted in a furnace gets a high damage value see MCPE-47907 for a better explanation.