Knocking her out at the Grove, even if she's temporarily hostile during the raid will not allow you to recruit her later, whether that is a bug I don't know in which way. ( whether she should be dead or available)
In essence, the "good" way to recruit Minthara is to knock her out at the Goblin Camp while she is temporarily hostile, and then make sure you don't long rest until you complete the related quest, not later.
Is how the non-lethal works mechanically, not only for Minthara but throughout the whole game. If the NPC isn't temporary hostile, the game considers them dead if you knock them out.
