The mainhand item from another world visually appears in hand if you pause the game during world creation.
I left a world with a snow layer in my mainhand and then I created a new world. I opened another tab while the world was "generating". When I went back to the tab it was paused and I had a snow layer in my hand despite not having an item in my hotbar. It was just a visual glitch because when i unpaused the game it disappeared.
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However, we are already tracking this issue, and this report is a Duplicate of MC-9232, which is a report that has not been resolved yet.
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