Another of the several things missing in the log and resulting in misunderstandment, incomprehension, doubt. ...
As far as my limited xp goes, newly trained units the log message refers to are the non irregular and non militia units you levy from scratch. Those units start with a red ribbon and are ultimated after some turns (depending on unit type).
You get that message when training is nearly complete and this means you can move them. The manual states you had better not rush them in combat so soon. I don't think the re-training refers to these (they are already regular troops) but suggests you not to put them to fight as their cohesion value is not at its best yet.
Technically when you are rushing these newly born units into combat, you are moving them...and moving them further affects their cohesion. So it's low cohesion troops losing cohesion when going to combat. Bad idea.
If you wait more, you'll see their cohesion increase but not their XP (I think theres'a leader trait that would help in this case).