This is actually super interesting: for displays that can get bright enough for HDR content, but do not have self-illuminated pixels/local dimming zones/any other mechanism to display very dark content alongside very bright (like, say, most modern iPad and Mac screens), this is exactly what is happening.
macOS/iPadOS cranks the backlight brightness up but then adds a black filter to the non-bright content to sim it back to “normal” levels.
Yeah, it seems to auto-dim the "SDR" brightness range a bit too much; the jump in the white background brightness ends up being clearly noticeable when e.g. I'm switching to an from the superbright QR code tab in the browser
