D3D12 is currently not being worked on. The person who originally implemented the backend has disappeared and none of the current graphics developers are willing to tangle through the mess that is the semi-finished backend.
Stenzek did take a look at it and fix a bunch of bugs before giving up; you could try this build - https://github.com/dolphin-emu/dolphin/pull/4094
Stenzek did take a look at it and fix a bunch of bugs before giving up; you could try this build - https://github.com/dolphin-emu/dolphin/pull/4094