Is it all stacks or 1? Because if it’s only 1 could it be that the 2nd stack is being applied as the first falls off, or 3rd applied as 2nd falls off. It doesn’t refresh the duration of previous stacks when you add another stack
Iv’e noticed it aswell when tanking with my druid, my logic told me that the stacks can fall off when taking damage but that might not be true as i havent read upon the subject.