Maybe I just missread. 
One of the devs actually contacted me and explained me the mechanic. Instantly getting my inconditional favor and granting my hate for all the people that trash them.
This is how it goes (hope I put it correctly):
- Fire works with an instant damage when you enter a fire tile, or lit on fire, and a status effect starting to add damage on following turns (effect of tiles delayed 1 turn).
- Moving from a fire tile does not produce damage (removed)
- Fire effect is inherited from the tile you stand on and this is reduced from 40 by 10 per turn (the fire on the tile itself is reduced). Fire effect is instantly removed when you walk out of fire.
My thoughts?
- Being on fire should severely reduce the perception or accuracy. Because it makes sense and because otherwise long range enemies on fire can just stand still and shot at you with little to no penalty. Making the use of fire only suitable for short range targets and kind of pointless.
I think a difference as small as this one will completely change the way I feel towards the mechanic.