If you use a buff behavior with a maximum stack count of 2 or more and replacement set to lowest duration, then upon reapplying the buff the incorrect stack gets replaced.
as far as i can tell it always replaces the first stack, resulting in the rest of the stack expiring although applying new stacks.
this is even more visible if the extend flag is set, resulting in the first stack being extended by all additional apply behavior effects until the 2nd (or higher) stacks expire