Behavior reproduced in 4.17.2 CL 3658906, 4.18.3 CL 3832480 and 4.19.P4 CL 3873914
To run the engine with DX12 I made a shortcut of the UE4Editor.exe found at D:\Epic Games\UE_4.18\Engine\Binaries\Win64 and added -DX12 to the target
Result: Editor Crash
Expected: Flat tessellation will be applied to material
Fatal error: [File:D:\Build\++UE4+Release-4.18+Compile\Sync\Engine\Source\Runtime\D3D12RHI\Private\D3D12Util.cpp] [Line: 242] (Device->*TPSOFunctionMap<TDesc>::GetCreatePipelineState())(Desc, IID_PPV_ARGS(&PSO)) failed at D:\Build\++UE4+Release-4.18+Compile\Sync\Engine\Source\Runtime\D3D12RHI\Private\Windows\WindowsD3D12PipelineState.cpp:582 with error E_INVALIDARG UE4Editor_Core!FDebug::AssertFailed() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:414] UE4Editor_D3D12RHI!D3D12RHI::VerifyD3D12Result() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\d3d12util.cpp:242] UE4Editor_D3D12RHI!CreatePipelineState<D3D12_GRAPHICS_PIPELINE_STATE_DESC>() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\windows\windowsd3d12pipelinestate.cpp:582] UE4Editor_D3D12RHI!FD3D12PipelineState::Create() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\windows\windowsd3d12pipelinestate.cpp:629] UE4Editor_D3D12RHI!FD3D12PipelineStateCache::Add() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\windows\windowsd3d12pipelinestate.cpp:288] UE4Editor_D3D12RHI!FD3D12PipelineStateCache::FindGraphicsLowLevel() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\windows\windowsd3d12pipelinestate.cpp:241] UE4Editor_D3D12RHI!FD3D12PipelineStateCache::FindGraphics() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\windows\windowsd3d12pipelinestate.cpp:194] UE4Editor_D3D12RHI!FD3D12DynamicRHI::RHICreateGraphicsPipelineState() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\d3d12rhi\private\d3d12state.cpp:336] UE4Editor_RHI!RHICreateGraphicsPipelineState() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\rhi\public\dynamicrhi.h:929] UE4Editor_RHI!GetAndOrCreateGraphicsPipelineState() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\rhi\private\pipelinestatecache.cpp:1044] UE4Editor_RHI!SetGraphicsPipelineState() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\rhi\private\pipelinestatecache.cpp:177] UE4Editor_Renderer!CommitGraphicsPipelineState<FDepthDrawingPolicy>() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\public\drawingpolicy.h:271] UE4Editor_Renderer!FDepthDrawingPolicyFactory::DrawMesh() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\depthrendering.cpp:742] UE4Editor_Renderer!FDeferredShadingSceneRenderer::RenderPrePassViewDynamic() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\depthrendering.cpp:849] UE4Editor_Renderer!FDeferredShadingSceneRenderer::RenderPrePassView() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\depthrendering.cpp:986] UE4Editor_Renderer!FDeferredShadingSceneRenderer::RenderPrePass() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\depthrendering.cpp:1300] UE4Editor_Renderer!FDeferredShadingSceneRenderer::Render() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\deferredshadingrenderer.cpp:775] UE4Editor_Renderer!RenderViewFamily_RenderThread() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\renderer\private\scenerendering.cpp:2081] UE4Editor_Renderer!TGraphTask<`FRendererModule::BeginRenderingViewFamily'::`26'::EURCMacro_FDrawSceneCommand>::ExecuteTask() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\public\async\taskgraphinterfaces.h:784] UE4Editor_Core!FNamedTaskThread::ProcessTasksNamedThread() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\async\taskgraph.cpp:651] UE4Editor_Core!FNamedTaskThread::ProcessTasksUntilQuit() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\async\taskgraph.cpp:560] UE4Editor_RenderCore!RenderingThreadMain() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\rendercore\private\renderingthread.cpp:327] UE4Editor_RenderCore!FRenderingThread::Run() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\rendercore\private\renderingthread.cpp:461] UE4Editor_Core!FRunnableThreadWin::Run() [d:\build\++ue4+release-4.18+compile\sync\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:76]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-55127 in the post.