Alright! Got a solution. Seems it’s the audio tracks are causing the problem. I was able to demux (and got that error again at 99.4%). Then I successfully muxed (to 100%) a BD iso using just the video and only one audio track (the first one). I’m gonna repeat the process muxing by adding one track at a time until I find the culprit, then just leave that one out of the final iso. It’ll take a few days probably, but I’ll report back when I know which track is causing my issue in case it helps anyone else.