Nasus' Wither unexpected interaction when losing vision of target

So, I just had a game in which i was targeted by the opposing Nasus's W - Wither with my Red Trinket active and standing in a brush. The enemy team had vision on me due to a ward in that brush, so Nasus tried to hit me with his W.

While he was in the cast animation he lost vision on me, so i wasn't affected by it at all however. Now, i did check the replay after the match and it turned out that he wasn't negatively affected (no Mana spent, ability did not went on cooldown).

The reason I'm still posting here is, that I expected a different result from this interaction. Since he already started the animation and there was the indicator of mana cost, I should've been affected by the ability at that point.

I recorded the moment of the match, so you can see what happens. In case this is the intended way of the ability to work, then my apologies for making this topic, but for now, I expect it to be a bug.

Link to the video: https://youtu.be/IirPs6xEWgo