HDR playback relies on the source, player, output format and display handling the same dynamic-range path; a mismatch can change brightness and colour.
Why this happens
An HDR label describes more than the number of pixels. A source uses a particular brightness and colour representation, and the playback chain needs to interpret it correctly. The stream, player, connected receiver, HDMI route and television settings can all matter. A dark or flat picture therefore does not automatically mean that the source is low resolution. It may also be difficult to compare fairly if one input uses a different picture preset from the other.
Start with the supported format information for the actual streaming device and television. Compare an ordinary SDR source with the affected HDR source while leaving unrelated picture adjustments alone. A content-matching option may help on a platform that documents it, but an arbitrary forced HDR mode is not a universal improvement. If you change a setting, keep its original value. Note whether the problem occurs only with one format or through one receiver connection; that is more useful than turning every brightness control to its maximum.
A useful way to work through it
Separate the source HDR format from the TV panel resolution.
Compare a known SDR programme and the affected HDR item on the same input.
Use the device and TV instructions for supported dynamic-range/output settings.