Fire created by lightning will stay on forever even if rain is pouring down over it. It can only be extinguished manually, but doesn't affect players.
MC-24
Ghost Fire caused by lightnings while game rule doFireTick false
Resolved
Duplicate
5
Rodrigo Serna
(Unassigned)
Unconfirmed
Minecraft 1.4.1
Retrieved
I can confirm this, It happens to me all the time.
The only way to get rid of the flame is by placing a block on top of it.
Its basically a ghost flame (you can't get injured from it)