I usually adopt all my generals as my Daimyos and their sons seem quite incapable of reproducing themselves (this really does need fixing) but have still had more than one desert to the enemy.
Can't remember if any were heirs though - probably not given that my heirs are mostly children and all seem to get killed as soon as I send them out of the capital.
However if you remember MTW it was almost compulsory for heirs to revolt and take half the provinces with them - at least in STW2 they just seem to take the army their with (and not even all of it - I am sure that one of my defectors took half his stack and immediately attacked the other half).
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