Diagnosis
This is usually not a defect, but a mismatch of settings. The TV might be in an incorrect picture mode for HDR content, or a power-saving feature is overriding the brightness.
How to Fix It
First, when you're playing HDR content, press the Home button and go to Settings > Picture. Ensure 'Picture Mode' is set to 'Dynamic', 'Standard', or 'Movie'—'Natural' and 'Filmmaker Mode' can look dimmer. Most importantly, scroll down to 'Expert Settings' and make sure 'Contrast Enhancer' is set to 'High' and 'HDR Tone Mapping' is set to 'Active'. Also, turn 'Auto Brightness' (an ambient light sensor) OFF. These are the key settings that pump up the brightness and pop for HDR.
If That Didn't Work
The issue might be with the streaming device itself. If you're using a built-in app, try this: go to Settings > General & Privacy > Power and Energy Saving and set 'Brightness Optimization' to OFF. If you're using an external device like a Fire Stick or game console, check that its HDR output settings are configured correctly—sometimes they send a weak HDR signal that the TV misinterprets.
Heads Up
This TV doesn't get as bright as higher-end QLEDs. In a very bright room, HDR will always look a bit washed out. It's a limitation of the panel, not your settings. For best results, watch HDR content in a dimmer environment.