Slow damage affliction

Hi, when is damage supposed to be calculated, because if heals come first then damage is ignored and doesn't apply on some games with some champions. Katarina ult over 1.5 seconds should be 800 damage, but instead I got less than 200 without much magic resist. The healing by the champion block over 600+ health somehow? I think the damage was forgotten about because of Riot spaghetti code. We all believe this so fix it maybe?