It seems that when you right-click on the hotbar while in creative mode with any vanilla armor, it will equip and disappear from the hotbar to be placed in the armor slot. However, this doesn't happen with custom armor. Instead, it appears to duplicate.
Thank you for your report!
We're tracking this issue in https://report.bugs.mojang.com/servicedesk/customer/portal/2/MCPE-159736 , so this ticket is being resolved and linked 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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