https://www.overclock3d.net/news/gpu_displays/amd_clarifies_freesync_2_hdr_s_display_requirements_following_controversy/1
 
AMD claims that all FreeSync 2 HDR-certified displays exceed DisplayHDR 400 requirements, but not all meet the DisplayHDR 600 minimums. In such cases, monitor manufacturers may stick both DisplayHDR 400 and AMD FreeSync 2 HDR logos in their specs-sheets or the product itself, but that doesn't mean that their monitors can only put out 400 nits brightness. It makes sense to me but might confuse others.
 
AMD's official statement: "The FreeSync 2 (now FreeSync 2 HDR) specifications were set almost a year before the VESA DisplayHDR standards were published. These two programs are separate and independent from each other.
When DisplayHDR 400 was defined, it was clear from the start that the FreeSync 2 requirements for color gamut, max brightness and contrast ratio set a higher bar than DisplayHDR 400. AMD is not lowering the bar for FreeSync 2 HDR to align with DisplayHDR 400. We're clarifying that a display that meets the requirements for DisplayHDR 600, or higher, could meet the color gamut, max brightness and contrast ratio requirements of FreeSync 2 HDR. FreeSync 2 HDR also has additional requirements for gaming and usability in areas not covered by VESA's DisplayHDR specifications.
We want to ensure at least 2x the perceivable color gamut and dynamic range than an SDR display as we stated from the initial announcement of the FreeSync 2 program. DisplayHDR 600 minimum specifications align with this objective, DisplayHDR 400 minimum specifications do not.
It is possible for a display to meet the FreeSync 2 HDR requirements but fail the DisplayHDR 600 minimums. Such a display may have the DisplayHDR 400 logo and the FreeSync 2 HDR logo, but it would be exceeding the minimum requirements of DisplayHDR 400."