That's an intended mechanic. The Unstoppable buff on the opponent allows them to complete their attacks without being interrupted. Rhino goes unstoppable when he's dashing at you, so you don't wanna intercept him. Ideally, it's best to wait out the Unstoppable buffs on champions, otherwise you're gonna be comboed into oblivion. Cap IW, She Hulk, Spider-Gwen, Stealth Suit Spidey can ignore Unstoppable. Good luck!