Vi's Ult Interrupted After Cast [Gameplay]

During a game on Summoner's Rift (5v5 Ranked) the Vi on my team used ASSAULT AND BATTERY against an enemy champion. After cast (the travel path line and target indicator both became visible and she started her charge towards the champion), the enemy Skarner used IMPALE on her. According to the tooltip both in-game and on the League of Legends website "While charging, Vi cannot be stopped..." Skarner's ability should have either failed cast or gone on cooldown. Instead the ability activated, and while not suppressing her out of her ability, dragged her around (per the usual routine) until the ability time had completed. At this point, Vi continued her Ultimate. Being she can still receive damage during her Ultimate, this is a significant oversight in the mechanics of the ability (I have yet to try other suppression abilities from Warwick and Malzahar, but I would assume they would act similarly).