There is currently an issue effecting /gamerule dofiretick. If fire is either created or in existence when firetick is set to false. The fire will stay in the state of firetick false even if firetick gets set back to true. This seems to have been in issue since 20w12a
Thank you for your report!
We're actually already tracking this issue in MC-179890, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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