What, exactly, did you observe? Because here's how it should work:
Shoal has two versions of her kiss. One drops the target to 1 HP and briefly knocks them unconscious on a failed save, one kills the target outright on a failed save. While these are technically damage effects, damage on the character doesn't make that first one any more lethal. The only way that's going to kill you is if you were poisoned or if you lose some bonus HP (barbarian rage wearing off?) shortly afterward.
If the target is the protagonist, Shoal always uses the first version of her kiss. If the target is a different party member, she rolls 50/50 on which version of the kiss to use.
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Shoal has two versions of her kiss. One drops the target to 1 HP and briefly knocks them unconscious on a failed save, one kills the target outright on a failed save. While these are technically damage effects, damage on the character doesn't make that first one any more lethal. The only way that's going to kill you is if you were poisoned or if you lose some bonus HP (barbarian rage wearing off?) shortly afterward.
If the target is the protagonist, Shoal always uses the first version of her kiss. If the target is a different party member, she rolls 50/50 on which version of the kiss to use.