DIAGNOSIS: Your TV is receiving an HDR signal, but the picture mode isn't switching correctly to an HDR-optimized setting. It's probably stuck in a Standard or Custom mode meant for regular TV, which makes HDR look flat and gray.
FIX: First, force the correct settings. While your PS5 is on and playing HDR content, press the Quick Settings button, then go to Picture > Picture mode. You should see options like HDR Vivid, HDR Game, or IMAX Enhanced. Select HDR Game for gaming. This applies the correct tone mapping. Then, go into Advanced settings and ensure HDR Tone Mapping is set to Gradation Preferred for brighter highlights, or Brightness Preferred for overall luminance.
IF THAT DIDN'T WORK: Check the PS5's own settings. On the PS5, go to Settings > Screen and Video > Video Output. Make sure HDR is set to On When Supported. Also, under 4K Video Transfer Rate, try setting it to -1 (Automatic). Sometimes a bandwidth hiccup can cause the TV to fall back to a non-HDR signal.
HEADS UP: The X90L has two HDMI 2.1 ports (3 and 4). For best PS5 performance with 4K/120Hz HDR, you must use one of these ports and ensure the HDMI signal format for that port is set to Enhanced (VRR) in the TV's external input settings.