Items not stacking in the inventory, if you double click on the item. You have to tap on each of the same item and drag it to stack it or throw all the items and pick them up to have them stacked together.
Items can be separated in the inventory but not stacked, it also applies to all forms of storage in game.
Thank you for your report!
We're actually already tracking this issue at MCPE-66458, so I resolved and linked 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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