At some times when an item has been smelted on a furnance, the arrow appears as the furnance still smelts, but in a stationary phase. You can reproduce it by smelting 1 iron ore with a coal on a furnance, exiting the furnance's UI antil the iron is smelted and the open it again. You will observe this bug.
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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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