Burning TNT

You have disabled firespread through

/gamerule doFireTick false

.

no
i only disabled mobGriefing

Unable to reproduce then.

Well, it happened to me but I tought it wasn't a bug
I'm uploading an image of how I did it
There is netherrack under the wood and water under the TNT to prevent the explosion destruction.
As the fire is spreading, sometimes it starts above and/or the sides of the TNT. Sometimes the fire vanish without lighting the TNT!

it haappens when you light the block above the tnt