This might be more of a "to do" than a bug since I know there is some Qt issues going on right now with 5.12 on master. I just tried downloading the Krita Next build for Windows 10 (2/24/2019 version). I am not seeing the new HDR painting stuff in the Configure Krita > Display area. It is in the code diff, but it looks like the feature is hidden in the UI if the HDR patch wasn't applied to the custom QT build.
No, but Dmitry hasn't merged the hdr branch yet, and soo I haven't managed to rebuild the deps yet... We won't forget about merging it, though :-)