Slitherine

Recently I noticed the following while playing allies resurgent and allies victorious:
In a couple of scenarios, the ai is retreating heavily damaged units (using the exit hexes to leave the map and not to be redeployed). As far as I noticed this is the case, while there is no scenario objective for me to prevent this (f.e. in the dday scenario).
Now it makes sense in terms of immersion but I wonder how it is working in terms of gameplay. Does the ai keeps the units for the following scenarios (that would be great but I don't think so)? If not the only reason for the ai would be to use the cp points (assuming there is a cp limit for the ai). But if this is the case it would be better not to destroying units that are going to leave the map.
Does anybody know how it works?

I'm fairly sure AI units don't pass on later scenarios unless there's a trigger that checks if a certain unit exits the map, and if it does it sets a campaign variable. A later scenario then checks the variable and if fulfilled it creates an identical one in a later one.