It's better to create a new thread, instead of permanently kidnapping and updating a solved one.
Fair point. I misread initially when going through this thread, and mistakenly concluded that it was marked as 'resolved' because the author had simply downgraded. I now see that was someone else.
Thank for your suggestion. I will open a new thread now.
EDIT: Pending moderator approval, there is a new thread: Noise / artefacts when playing back 4K HEVC video on RPi4 + LE 9.2.1 (no problems on LE 9.2.0)