The system does not know why you left, say if you pulled the plug on your internet and waited to be kicked, or you were being toxic and the group did not want to continue with you. So unfortunately you get the debuff if you left on purpose, or got kicked.
It used to be you could avoid the debuff by completing a smaller portion of an instance, managed 1 boss, or stayed for a bit of time.
Unfortunately feedback on people leaving led to the new changes,