Do the barbs control any cities when this happens? When barb city capturing is enabled, the mod runs the normal econ logic for the barb player so they can have city production. That logic charges players unit maintenance as long as they have at least one city. I think what's happening in your case is the barb player is going bankrupt from maintenance on all the spawned barbs and then those units are getting disbanded.
Yes, they have one city now, at least in N. America on a large/huge earth map. They also have a lot of camps too. In the game I first noticed this, I played long enough to where they should've been spawning their advanced units, but they seemed to stop spawning altogether; and, yes, they had a city or two.
A unit maintenance penalty would certainly explain both the disappearance and non-spawning of barbs. Come to think of it, I think I recently altered the 'bankrupt' setting; it's a long shot, but I'll try changing it back to see if there's a difference.
I should be able to fix this by making units maintenance free if they have a barbarian tribe ID set, which should cover barb units that were spawned in but not any units built in cities. In the mean time, since you're playing your own scenario, you could create special unit types just for the barbs and give them the king flag so they're maintenance free.
Okay, thanks for the workaround. If my above experiment proves fruitless (probably), I'll follow your suggestion to, hopefully, keep the barbs relevant longer. Also, as you suggest, making them maintenance-free in a future patch would seem a sensible fix. Although they capture cities, they don't seem to have any workable tiles, and so paying maintenance for non-city barbs is a bit crippling.
Other than this issue, is barb city capturing working properly? (Anyone who's played with it, please share.) I marked it as experimental but never got much feedback so I don't know how well it works.
Overall, I think what you've done for barbs is great; they're a true background threat that every civ has to contend with. I'll keep you informed on either/both of the above changes if they make a relevant difference.
The idea you proposed earlier of being able to set more advanced units (vs. only one now) would contribute to them being a potential factor for longer. If you don't mind another suggestion: when the barbs capture a city, it'd be cool if they had workable tiles -- again, making them more relevant, combined with your idea, to advance enough to manufacture more and stronger units. But maybe you've already considered and discounted this pragmatically.