Per UDN Licensee:
I have 2 crash repro when enabling DirectX12, both linked to tesselation and still present in 4.18 stock engine and easy to reproduce.
Crash 1
-Start editor with DirectX12
-Create new material and open material editor on it
-Select 'Flat Tesselation' in tesselation settings
-Crash
D3D12 ERROR: ID3D12Device::CreateGraphicsPipelineState: When using tessellation, the primitive topology type must be D3D12_PRIMITIVE_TOPOLOGY_TYPE_PATCH [ STATE_CREATION ERROR #668: CREATEGRAPHICSPIPELINESTATE_HULL_SHADER_INPUT_TOPOLOGY_MISMATCH]
D3D12: *BREAK* enabled for the previous message, which was: [ ERROR STATE_CREATION #668: CREATEGRAPHICSPIPELINESTATE_HULL_SHADER_INPUT_TOPOLOGY_MISMATCH ]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-53352 in the post.