[Bug] If vayne tumbles during Warwick's Ultimate she can avoid lethal damage

When I play as Warwick and Ult a Vayne she can avoid damage by using one of her abilities. I assume it is her Q. Before I explain what happens I want to clarify this happens when Vayne is at any health AND specifically when Warwick's Ultimate alone has enough damage to completely kill Vayne with just the damage from the Ult. What happens is.. I click on her with my R.. Warwick starts his suppression.. Then Vayne goes invisible mid suppression somehow.. The Ultimate ends and within 1 sec Vayne reappears.. the damage she has taken is less than what she should have received.

This has happened countless times and it seems like Vayne can do this with or without activating her Ultimate.

It is quite troublesome when I go in to team fight and I Ult Vayne when she is at a health were I can clearly 1 shot her with my Ult and I see her Health bar melt and then I go on to attack someone else and a second later she reappears and starts attacking my team again.

No other champion can avoid my Ult damage unless there abilities or passives specifically allow them to cancel Debuffs.