emmmm......To be honest ,I do not know ,My server members reported that they had never seen anything like that before...but I had seen similar situation very long ago...maybe a edition of 1.12 or 1.13,in a BDS of China Netease version,too.When I download the world ,it was still there.
The technical operator of our server told me that this kind of fire is caused by the change of value of dofiretick: When the value changes from True to False and the burning blocks were broken,the fire will be invisible,but it will not go out,even the value changes back to True. And now this area has found at least 10 more invisible flames which can not be put out.
And I downloaded the world, the flames are still there.