I did a little testing tonight on my E6 with my OPPO 203 set to forced DV. I then inserted the AVSHD test pattern disc into the 203. This will force all the patterns to be displayed in DV HDR. Much to my surprise, setting the brightness based on the brightness pluge pattern, a setting of 49 was correct with flashing bar 17 very faint. Then I went to the 0% full screen black slide, and confirmed 49 was correct. Raising to 50 produced a glow. A 0% black slide should look black whether it's a rec709 slide converted to DV or an actual 0% DV slide. Black is black. If you have an OPPO 203, or another player that can force DV on everything, and you put the AVSHD patterns on disc, give it a try and see if you have the same issue. Make sure the room is dark and at night. This also seems to have fixed the elevated DV blacks with the ATV4K and the elevated blacks with DV on the internal VUDU app.
LG 65E6P OLED - LG 65EF9500 OLED - OPPO 203 - OPPO 103D