It seems as though "Whole Group" acts just like "Any group member" when it comes to activating a trigger, and vice-versa. In my case, the trigger should activate when a whole enemy group isn't present, but the trigger activates as soon as a unit of that group dies.

This is a tricky situation as many mission editors must've used "any group member" when they originally meant "whole group" (and vice-versa) as a workaround, and correcting this bug can and probably will break some missions, but it still needs to be fixed. {F26161}