It seems like there's something wrong with the selection of picture settings on my PS5. As you can see, I have to click on "Cinema Home (user settings)" twice before it goes to the correct brightness.
When I apply "Filmmaker Mode (user settings)" as advised in • LG G3/C3/G4/C4/C5/G5 OLEDs Optimized Setti... , the picture is correct for a few seconds. After that, the picture dims conspicuously as if the PS5 hasn't been used for a while.