ahhh! Got some lit!
https://www.sciencedirect.com/science/article/abs/pii/S0019103510004641
This is a pay-for article, but it seems that CO2 not happy on Iapetus. Won't trap for long.
What CO2 there is seems associated with low albedo stuff. (Not where youd expect from cold trapping)
So CO2 is either being generated from UV action of exposed organics/ice (in dark lag+UV produced superoxides from ice photolysis) or possibly-maybe CO2 got trapped up by associations. mineral/carbon stuff in dark lag material.