[GAMEPLAY] Grag wasn't knocked up by Malph R

When playing as Malphite, I ulted into 4 enemies and Gragas E'd into my destination point. It was like he had Banshee's Veil. He wasn't knocked up or damaged and his Body Slam landed on me the exact instant I was no longer unstoppable. My ult should have hit all five enemy players.

To reproduce this bug:

1. Get Malphite in R range of a group of 4 enemies standing close together, and keep Gragas outside the radius of Malphite's impact Zone.
2. Use Malphite's ult on the group. Just before Malphite impacts, use Gragas to E into Malphite's destination point.

Expected Outcome: Gragas's E is interrupted, Gragas takes damage and is knocked up. Actual Outcome: Gragas's E is not interrupted, Gragas takes no damage and is not knocked up. Gragas's E lands on Malphite.

Screenshots

Me as Malphite still travelling: https://ibb.co/R0fYBfC

Gragas's E landing on me after the knockup starts: https://prnt.sc/lvquua

Game link: https://matchhistory.euw.leagueoflegends.com/en/#match-details/EUW1/3866381001/2085764997238944?tab=overview