bdebaere

There is no loot set for this NPC. Execute the following SQL statement on your mangos database to fix:
INSERT INTO
creature_loot_template( entry, item, ChanceOrQuestChance, groupid, mincountOrRef, maxcount, condition_id )
VALUES
(2714, 4482, -100, 0, 1, 1, 0);
UPDATE
creature_template
SET
LootId = 2714
WHERE
entry = 2714;

Right-clicking the 10 Witherbark Totem Sticks you cast a spell which creates the gameobject Witherbark Totem Bundle (174764) above the stone. This gameobject however is not lootable.

Interestingly there is a gameobject with the name Bolt Charged Bramble (103662) from which you CAN loot the item with the same name. This gameobject however is not spawned by the spell (9055) with effect_1_misc_value of 174764.
There is also another spell in the DBC right after 9055, 9056. This spell summons the actual Bolt Charged Bramble (but not above the stone like it should), lootable and ready. This spell only consumes 1 Witherbark Totem Stick, is instant cast, and does not require the stone.

So we have the spell (9055) which summons an unlootable WitherBark Totem Bundle (174764) object and requires to be near the stone and 9 Witherbark Totem Sticks. And the spell (9056) which summons a lootable Bolt Charged Bramble, which requires 1 Witherbark Totem Stick. Only thing left is to find a way to put both spells together.

The spell that is triggered by this Idol (id: 22398) is 27853.
effect_1_misc_value: 4415

This is not an issue with Frost Nova but with the NPC in question: Scarlet Vanguard.
I have tested Frost Nova 50 times on other creatures in packs of 5-10 and did not get the same result.