Tahm Q+W Combo Not Using Enough Mana

I'm not sure if this is intentional or not as it doesn't say in the tooltip, but activating W while Tahm Kenches Q is in midair, and landing the Q on the minion, causes only the combo to cost the mana of the Q (I think around 50) instead of the full Q + W amount (around 130/140) Sorry if this is intentional or already know, just posting because it's really easily exploitable while laneing as Tahm