That really sounds like an oversight on Larian's part. A paladin probably shouldn't be able to kill anyone in the grove without breaking their oath, just like you can't kill the non-hostile goblins in blighted village
I think you could maybe even argue that the oath should be broken as soon as you agree to support Minthara in the shattered sanctum, as either you are lying or you genuinely intend to kill innocents. Though it feels as though it would have more dramatic impact later, so actually it would maybe it should happen as soon as you choose the option to side with Minthara at the grove.
Agreeing to support the tieflings after all is a trickier one. Yes, if the paladin had lied to Minthara then that would be breaking the oath (the ability to distinguish this case might be another argument for making agreeing to support Minthara when you don't intend to keep it a deception check, which I've said elsewhere that I'd like anyway for roleplay purposes and also to help signpost that there will indeed be a chance to go back on the promise later).
But if they agreed to help honestly, would that straight away break their oath? Intention isn't the same as action. And if intending to do something that breaks your oath doesn't immediately break it, then there is still time to have a long, dark night of the soul, or just not be able to follow through when the moment comes, and decide after all to do the right thing. So while turning against Minthara at the gates is breaking a promise to her, it feels a bit odd that breaking a promise to do an evil thing should break the oath when the promise itself didn't. After all, the paladin would already have been trapped on a path where the only outcome would be a broken oath ... though now I put it like that, maybe that's an interesting story.
Or maybe that's a reason to have the oathbreaking back in the shattered sanctum after all, though then you might actually break your oath there by lying or promising to do an evil thing, pay to redeem it, then break it again by either colluding in the slaughter of innocents or breaking a promise to do an evil act (if the latter does indeed constitute breaking your oath).