DIAGNOSIS This is almost always caused by the TV's Auto Brightness Limiter (ABL) or Auto Static Brightness Limiter (ASBL) kicking in aggressively, combined with incorrect console settings. These are safety features to prevent static image burn-in, but they can overreact.
FIX First, on your PS5, go to Screen and Video settings and set the HDR calibration using the system tool. Then, on the LG TV, press the settings button, go to All Settings > Picture > Advanced Settings > Brightness, and turn OFF 'Energy Saving Step' and 'Auto Dynamic Contrast'. Also go to 'OLED Care' > 'OLED Panel Care' and set 'Adjust Logo Brightness' to 'Low'.
IF THAT DIDN'T WORK The issue might be your HDMI cable. Ensure you're using a certified Ultra High Speed HDMI cable. A subpar cable can't carry the full HDR metadata, causing the TV to default to a dimmer tone mapping. Swap it for a known-good, short cable.
HEADS UP In very bright HDR scenes, some dimming is normal to protect the panel. If you disable all protection features for max brightness, you significantly increase the risk of permanent image retention (burn-in) over time.