Fire will spread up when the only block touching it is the one it is on top of.
Comments 2
Cleaning up old tickets: This ticket has not been updated recently (~1 year+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines.
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It's not clear from your description what it would be spreading to. Please provide a screenshot or give more details.