DIAGNOSIS That's Game Mode doing its job—it turns off a lot of picture processing to reduce lag, which includes disabling some dynamic contrast and local dimming features. On the QN85C, this can make the image look flatter and dimmer by default.
FIX You need to recalibrate the picture while in Game Mode. With your PS5 on and Game Mode active, go to Settings > Picture. First, make sure your PS5 is set to output in "Game" picture mode (it should auto-switch). Then, adjust these: Turn "Contrast Enhancer" to High or Medium. Bump up the "Shadow Detail" a notch or two. Most importantly, go to Expert Settings and set "Local Dimming" to High. This restores a lot of the pop without adding significant input lag.
IF THAT DIDN'T WORK Check your HDMI input settings. On the TV, go to Connection > External Device Manager > HDMI Settings and make sure "Input Signal Plus" is ON for that specific HDMI port. This enables the full bandwidth for HDR and VRR. Also, on your PS5, check the HDR calibration in system settings to make sure it's not set too low.
HEADS UP Game Mode picture settings are separate from other modes. Tune it once while playing a game you know well, and those settings will stick for that HDMI input. Don't use the "Dynamic" or "Standard" picture mode in Game Mode—they'll still look off. Stick with "Game" or "Movie" as your base.