The 2020 Samsung QLED TVs have a very well known bug with local dimming. More info here. To reproduce this bug:

1. Reset TV to factory settings.
2. Enable HDR for HDMI4 (this is "input mode plus" in the settings).
3. Disable ambient light detection.
4. Disable anynet+.
5. Set local dimming to "standard".
6. Connect an HDR source to HDMI4.
7. Power the TV off then on, and start a timer.
8. At exactly 6 minutes, you'll see the screen get much darker. This is equivalent to local dimming being reset from "standard" to "low" (even though it's actually still set at standard). In other words, the TV stopped respecting your chosen setting for local dimming.
9. If you open the settings and change local dimming to "low" and then back to "standard", the setting will start working again, but again it will only work for exactly 6 minutes. So, if you want to watch a 2 hour movie and keep local dimming on "standard" the whole time, you'll need to open the settings menu every 6 minutes... no less than 20 times in those 2 hours!
10. As a sidenote, there is a related bug where even with non-HDR content, the screen will randomly dim suddenly. E.g. when watching a 2-hour movie, there will be a small handful of moments where brightness drops dramatically in the middle of the scene (this is harder to reproduce, since it does not happen reliably at 6 minutes like the one described above).

Samsung has the ability to fix this bug with a firmware update. In fact, they supposedly did fix it at some point in the past, but presumably there was a regression in a later firmware update, since the problem does still exist today (July 2022) on the latest firmware.

Please Samsung, fix this bug once and for all. It makes HDR content viewing unusable. This is inexcusable on a flagship TV.