What's the LG model? OLED? A couple of years ago LG changed the firmware and intentionally made it look rather poor in Game Mode. They released the following statement:
"Regarding HDR Game mode and darkness – a recent firmware update is based on changes to the gradation and saturation settings for HDR Game Mode, to ensure a better picture quality in HDR game mode. To implement this, the gradation was enhanced and the saturation was reduced. In the previous version of HDR Game Mode, the picture settings were similar to the Standard Mode settings, offering brighter images. Compared to previous picture settings the picture may look darker, but in fact, the new PQ setting brings improvements in terms of HDR gaming experience. For users who prefer a brighter image, they can adjust the Dynamic Contrast setting of Game Mode.”
You can overcompensate for this by turning Dynamic Contrast and Brightness up.
Alternatively, and depending on your model, try opening "Quick Settings" on your LG TV, select "Picture Mode", and then "HDR Effect" mode. You can then adjust HDR content from Light, Medium and High.
The actual reason I believe they changed it all is because they were worried about screen burn-in.
Some games such as Resident Evil 2 are just rubbish with it though.