It's not converted to DCI-P3. It just lets you know that the content was mastered to DCI-P3 using a BT-2020 container. So you are correct to use a BT2020 calibration.
I suggested to Madshi to change the sign he uses to convey this information, as I think it is confusing to many users, but for some reason he didn't think it was confusing so it's stayed like that because he is the boss
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It would be nice if people could refrain from making blanket statements such as this. Yes, new drivers often break things for some. Yes, sometimes they break things for the majority.
But as far as I'm concerned, the latest 390.x drivers work fine, at least in 4K. I'm set to 4K23p with a custom refresh mode created by MadVR giving me a frame drop every 1-2 hours, and I use RGB 12bits 4:4:4. When I play 4K60p, the driver swaps automatically to 8bits 4:4:4. That survives a reboot and works fine here, as it has always done.
There are two limitations that I am aware of:
1) Like all the drivers after 285.28, you have to select 12bits from a non custom refresh mode. Once a custom refresh mode is selected, the resolution is greyed out and can't be changed, but it's still 12bits in 23p and 8bits in 60p if 12bits was selected from a non-custom refresh mode (at 30p max).
2) Compatibility is broken with Asio4all and most Asio drivers. Thanks to a kind member, I was able to find Flexasio, which works fine with some limitations.
So instead of making blanket statements simply because it doesn't work for you, please post details about your rig (OS version and build, GPU model, driver version) so that we can see if there is a common link between those for whom 390.x works fine, and those for whom 390.x doesn't work.
My rig is detailed in my sig.
By the way, 391.24 was just released today, I'm about to try it.