It’s real nice of fire to not spread (or even go out) if you leave it alone but damn if it makes any sense.
This a design choice or a long forgotten oversight?
Where did you set the fire? Fire will spread if set on a flammable floor or surrounded by trees/other flammable materials, and it will go out after a period of time. If you set it far away/on another level and came back to find it untouched that’s more an issue of the reality bubble, where nothing happens outside of a certain radius of your character.
Does fire still operate if outside of the ‘reality bubble’?
Not sure about the same level, I’ve had faraway buildings on fire that eventually went out on their own, but I’m not sure if I was close enough to activate it or not. Definitely not for different levels, though. Set a lab level on fire and escape through the stairs immediately, and the fire won’t spread until you’re back on the same level.
The reality bubble is roughly 60 squares in all directions, inside that fire is active and updates every turn, outside that and it’s frozen until you get back in range. We have plans for tracking it so it will burn while you’re gone, but it’s a bit complicated.
Simple fix would be to extinguish fire when it leaves your reality bubble, though that would be weird.
Is that 60 map tiles or 60, you know, single tiles?
Just saying, in case you’re thinking about popping the big 1.0 anytime soon, it’d had to be addressed. 
[size=8pt]Oh shit, I wonder if that’s bad grammar. Kinda sounds like it.[/size]
We’re not considering popping the big 1.0 anytime soon.
IIRC, the next update will be 0.A. we’ll have to add a whole lot before we can have the audacity to call cataclysm 1.0.
NPCs in particular are one big initiative that people have been working on, and even now, they are still horrendously broken.
Since funnels now work regardless of the reality bubble, maybe the fire could be made to work in a similar way?
The difficulty in simulating fire when it’s outside the reality bubble is related to the simple fact that it can spread. As such this leads to a couple of different solutions with a couple of problems each. Here’s the basic list:
Solution: Fully simulate fire wherever it is until it burns out.
Problems: Once you start a forest fire or other large fire it might be a long time before the lag from it goes away.
Solution: Fully simulate only the submaps that have fire in them.
Problems: Fire will hit the edges of the submaps and stop, resulting in strange straight lines when the player returns. Also still some of the same lag problems.
Solution: Extinguish fires always.
Problems: Base catches on fire, player walks into the basement and walks back. Every fire is out and no damage has spread.
Solution: Wait until the player gets near to update a fire and retroactively find what it would have burned.
Problems: Player approaches old forest fire submap. Suddenly the entire forest flashes to ash (or a raging fire) around them when the old fire tiles are fast-forwarded.
The solution that I personally think covers the problem the best would be to occasionally reload fire maps and run a rough “fast-forward” style fire algorithm that is rougher but runs faster. That would mean large fires off-screen would still burn roughly as they would if you were there, while not providing too much lag in most cases.