See, this implication bothers me because Arenvald seems to have the surname Lentius, and if that's not Garlean I'll eat my circlet. I wouldn't even think of questioning it if not for that, but it seems VERY unlikely that a Garlean soldier would go to the trouble of marrying the mother if she was just some random local he assaulted, and even less that the mother would choose to so blatantly identify the man who abused her by giving the bastard his name, since that would not only be traumatic for her, but likely invite reprisal from the father.
This leaves me with a few possibilities:
1. Lentius is another 'Bastard surname' like Greystone, perhaps specifically denoting bastards of Garlean soldiers.
2. Lentius is actually 100% an Ala Mighan name, even though it sounds very Garlean.
3. Something was lost in translation here.
4. His parents were legally married for some reason, though I suppose there could still be reasons the relationship wasn't happy/consensual?
Maybe it's just me, but I'd find a love story between a Garlean soldier and one of the locals infinitely more interesting than yet another throwaway reference to how the horrible Garlean soldiers are abusing the womenfolk again.