I managed a bit more of free time and I’m happy to say that the fix for this issue has been found. We now know that CNUnknownFlag_100 has something to do with HDMI Audio. Adding this flag results in working HDMI audio. Tested on 2 affected systems and it seems to work. No further edits are required. Like, for ig-platform-id 0x3e9b0000 you would have to add 100 to the existing flags for index 0. The stock flags for index 0 were 0x98, after applying the fix the flags would be: 0x198.