When you take an item in the creative and place it twice in the same slot (where you craft, only), it does not come out at all (in the creative).
One possible solution for this is to leave your world and enter again.
MCPE-87961
Magnetic slot
Resolved
Duplicate
0
Patrizul
(Unassigned)
Unconfirmed
Android
Android 7.0
1.16.0, 1.16.1
Retrieved
Thank you for your report!
We're actually already tracking this issue at MCPE-73593, so I 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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