champs not taking dmg

very recently me and my friends have noticed that several games there have been players who gain hp or just stop taking dmg during team fights even gaining hp without attacking or healing ...kinda like the dmg was reversed from dmg to healing,,, good example is a vi chasing me with 50 hp and not dieing but gained half her hp while chasing me ....not possible in normal games. she also had no lifesteal or mr so wtf.... riot really needs to fix this crap these cheats are easy to find they are all over the net.