I don’t work for Twitch I cannot investigate/solve the problem. I just help look after the forums.
No one replied to your thread as they don’t have an answer, or there is no other information to add as you already linked a relevant thread with all the information we have
Sorry I can’t be of more use.
Every time I’ve tested SetVolume it has worked as expected, or as I have noted on the thread you linked or other similar threads. I can’t find a problem since browser interaction score gets in the way on other sites.