This answer your question more or less, you have to realize of this for some reason, or cause you play in a East - Asiatic FA so then you see yourself supporting in the 2nd leg when you were of shopping.... or you realize of this by your own for any reason as you did, but, since assos starting I never heard a person, here in the forum, or in 3-4-5-6 groups where I am discussing, or at any place, mentioning this like, "hey, I'm gonna attend the 1st leg, so at least I'll be auto supporting during the 2nd despite I'll not be there live".
Never saw this rule like this, as I said, I thought had logic to apply this once your own match started and in the same round, cause yes, sadly the sun rises for the east so the advantage is for Vietnameses that wake up first xDDD