IT is more than acceptable to push the limits, or go against the grain in certain areas. Take water changes for example. Generally accepted that you do them. There are people here that don't and have beautiful tanks. A while ago it seems, people couldn't keep SPS as well. Now every tank seems to have some. Without people attempting to do the impossible, the hobby doesn't advance.
HOWEVER, there are some generally accepted rules that won't change, because the fish don't change. A 75 gallon tank will NEVER be considered a proper home for 3 tangs. Its not a matter of technology, or maintenance, but pure size. Its like keeping a couple bull mastiffs in your 3rd floor 500 sq ft studio apartment in the city, just not a good idea and not good for the animal's long term health and well being. You ask why it would be unhappy, but "oh don't talk about the size of the tank". You can't watch the tank all the time, so can't fully disprove aggression. You haven't seen it swimming stressed, because its not swimming, its stressed and hiding.
Could it be other things? Sure, you made note at first that you have a very strict QT protocol, but it clearly isn't that strict, a fish went in without it. But a fish in hiding usually means stress, not disease. Not sure what more you'd learn by putting a fish in a tank ill-suited for it than you would by just asking "why isn't this a good idea".