I was just trying to enable HDR in URP - Unity does all the rendering and post effects in HDR, and the editor seems to run in HDR, but for compositing it seems to blit the HDR buffer into a R8G8B8 buffer... Looking at a feature matrix on Unity's website it claims that HDR output is only supported in the HDRP pipeline.
How did you get HDR output to work with URP? I just captured your demo in RenderDoc, and it confirms it uses an HDR buffer for the presentation.