Setting a block via setblock or fill to fire (regardless of age) neither spreads or burns out. 

Light a block on fire with flint and steel burns out as normal.

There does not seem to be any difference block states between manually setting a block on fire and setblock ~ ~ ~ fire.

Saving and closing world does not make the setblock'ed fire spread/burn out.

If I dump the region file for the specific chunk with a flint&steel fire and a setblock fire, the setblock fire doesn't have a block_ticks entry, whereas the one placed with flint and steel does;  

"block_ticks": [ { "i": "minecraft:fire", "p": 0, "t": 39, "x": 435, "y": 118, "z": -26 } ],