There is a post on Reddit that showed this bug: https://www.reddit.com/r/Vermintide/comments/rd2ddy/warrior_priests_implacable_has_a_bug_it_delt_the/

When the WP got the attack, he will get only 60% damage from the enemies at the beginning which is correct. However, in the next 3 sec, he will get 36% of the full damage instead of 20%.

I also did my test and it showed the same result.

First I got hit from the Ungor, which is supposed to be 25 damage, but I only got 15 because of this perk:

I started losing more hit points after getting the hit, now my hit points are 132/150:

I ends up with only 126/150 hit points. So I got 9 damage in the 3 secs, which is 36% rather than 20% of the damage I got.

By my understanding, I should only get 5 damage which is 20% * 25 in the 3 secs after I got hit. So the value here is wrong.