It’s worth noting that the percentages are not actually damage reduction in the typical sense and can be misleading. The percentage represents your effective HP increase from the resistances you have however they scale with your total health, the actually important bit for comparing is each protections value. It is sort of like a health pool that only works against one damage type but does not need to be healed.
In other words the higher your actual HP/ward total the lower the percentage will be for the same amount of resistance. Thus 50% ‘resistance’ on a 400 HP pool is really only giving you 600 effective HP (and the percent comes the actual protection value compared to your maximum HP) so if you stack poison up high enough or take a big enough single hit then you will 100 to 0 instantly. Similarly, if you stand in stacked AoEs then you can and will suddenly take a stacked tick of damage higher than your HP + Ward + Protection for that damage type.
Poison specifically can be a serious offender due to poison increasing 8% per stack so in a sense each stack is double dipping; increasing the damage due to adding more stacks but also increasing the damage from all existing stacks by that amount.