[GAMEPLAY] Mana miscalculation(?) on Tahm Kench's QW combo

I tested it many times in a Bot game, and it turns out that the cost does not equals to the total of both QW's mana costs. Tahm's Q costs 50 mana, and his W being 90 mana. When I used QW on a minion & monster, its cost is only 50 mana.

At first, I thought it would be a buggy mana-usage, as according to Lol Wiki:

While laning, Tahm should always be looking for opportunities to harass with Tongue Lash. He can also harass by using Tongue Lash on a minion immediately followed by Devour, allowing him to swallow and launch a minion as a projectile. While this does give Tahm some AOE damage, it also eats away a large portion of his mana and prevents him from using Devour defensively until it comes off cooldown, rendering the combo too expensive to be worth executing unless it is being used to clear a wave.

^ I guess it indicates that 50+90=140 mana being "large portion of his mana". Or could be a mistake.

Was there a bug in calculating his total QW mana cost? Or is there a missing tooltip that indicates that it only cost 50 mana to do a QW on monsters & minions? Red posts are highly appreciated :)

Pictures below as proof. I did made him finished casting his QW animation to recheck his last recorded total mana.