Disable Local Dimming on Samsung QN85A

Caveat: you may need to set the Device Type of your source to “PC” for this to work.

1. You need a remote with number buttons to bring up the service menu. You can use your smartphone to do that. Install the Samsung SmartThings app and add the TV into it.
2. Press “🔇 1️⃣  1️⃣  9️⃣  🆗”. The service menu should appear.
3. Press 1️⃣  2️⃣  3️⃣  4️⃣  to make the advanced menus appear.
4. Go to “Option” → “Production Option“.
5. Turn off “PC Mode Dimming Support“.
6. To exit the Service Menu, turn the TV off and back on.

Thanks to u/will_work_for_vinyl for their slightly different instructions which inspired mine.