In some TW games this is slightly modeled in, in the sense, that a unit might choose to fight till death, rather than running away, when they are surrounded and they have nowhere to run. So then you open up a way .. and they will try to run again, which is easier to finish them without own casualties.

But those are hardcoded algorithms.

The units do not really learn. (that would be awesome, but is very very hard to implement in realtime in a proper way) They just improve their stats with experience. So they get stronger in stats in either case, even when fleeing every single battle.

At least for Rome Total War and Medieval II, experience does mean a difference. A veteran army will fight for several times longer against overwhelming odds than a newly recruited one before breaking. It also depends on training: even green Roman troops will last longer in prolonged battle than green Gallic ones. Do not remember how later games deal with that. One thing the author of this series stresses is that battle is very emotional and you can't learn from a book that running away is a bad idea.