The Fury PvP talent Slaughterhouse cannot apply its stacks if a “stronger” (but really weaker) Mortal Strike effect is on the target. This is likely because the base effect of a single stack of Slaughterhouse is only 5% reduced healing, while other standard effects are 25% reduction. Slaughterhouse’s application should not be prevented in these scenarios, as it completely nullifies the talent when the warrior should simply be allowed to stack the buff to gain its benefit.
Additionally, if the warrior has already put 4 stacks of Slaughterhouse on a target (for 20% healing reduction), these stacks will be overwritten if a class with a 25% MS effect applies their debuff before the warrior can Rampage again to increase their own stacks.