if the fuel in a furnace is emty before the meat or ore is cooked the white part of the arrow usely slowly decrases but in the snapshot it diddnt eaven if the furnace is empty it still shows a partly completed arrow.
It is as far as I notice only an optical bug that doesnt changes the cooking because if you put something new into the funace the starts as normal.
Thank you for your report!
We're actually already tracking this issue at MCPE-63099, so I will resolve and link this ticket as a duplicate.
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