when fire spread is off, if you place a block and set it on fire, then break the block the fire will still be there, be unbreakable and continue to work as normal but be invisible.
Semi-fix: If you place a block under the fire, the fire will be visible and then you could break the fire and the block and then the fire will be gone for good.
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We're tracking this issue in MCPE-101371, so this ticket is being resolved and linked as a duplicate.
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