Yes someone posted a link at page 41 to the correct topic / bug report!
yay.
I was aready searching for a while when I made the previous post, it's not the explanation from the devs but it was confirmed not to be a bug.
https://beta.forum.elvenar.com/index.php?threads/bonuses-from-abilities-are-multiplied-wrongly.6584/
At least what I wrote was correct.
they use a slighty different way to calculate it but in essence it's the same.
I remember them explaining it like this way.
To calculate damage reduction they made it like this:
your base damage is 100
Then DR is added as a defence modifier = 180
Then a debuff is applied 180*0.6 = 108 which means 8% damage reduction stays.
In the above link they get the same result with the same math from a slightly different angle that might explain it better. but my version is easier to remember
But this magic math is why debuffs are so insanely powerfull a 40% debuff aint a 40% debuff but a much more massive one was it doesnt only debuff the base but also debuffs the DR.
This also explains why cannoneers are a joke on there first hit, and very painfull when a second cannoneer fires his shot. it does not make sense than a 30% debuff creates 4-5 times more casualties but with this magic it's all explained. this is why mortars are useless and 3* frogs where a revelation at that time. it's all about the correct debuff.
because if you have 80% defence vs heavy range and it does 1000 damage only 200 sticks and kills lets say 50 units.
Then inno magic happens 180*0.7 = 26% this means 740 damage is done and this kills 185 units 3.5 times as much.
off course the same magic happens but attack debuffs but thats just not as powerfull as. at best it's annoying when it happens to your units.