I have several patches of fire in the overworld that have persisted for several day/night cycles. They seem to be on just grass. I did notice a ton of lightning the other night, it's possible these are from lightning strikes.
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Thank you for your report!
We're actually already tracking this issue at MCPE-69072, so I will resolve and link 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.
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