When building a UWP app using the MRTK (https://github.com/Microsoft/MixedRealityToolkit-Unity), there are reference rewriter issues that end up looking like:

Error: field System.Numerics.Vector3 Windows.Perception.People.HandMeshVertex::Position doesn't exist in target framework. It is referenced from Microsoft.MixedReality.Toolkit.Providers.WindowsMixedReality.dll at System.Void Microsoft.MixedReality.Toolkit.WindowsMixedReality.Input.WindowsMixedRealityArticulatedHand::UpdateControllerData(UnityEngine.XR.WSA.Input.InteractionSourceState).
Error: field System.Numerics.Vector3 Windows.Perception.People.HandMeshVertex::Position doesn't exist in target framework. It is referenced from Microsoft.MixedReality.Toolkit.Providers.WindowsMixedReality.dll at System.Void Microsoft.MixedReality.Toolkit.WindowsMixedReality.Input.WindowsMixedRealityArticulatedHand::UpdateControllerData(UnityEngine.XR.WSA.Input.InteractionSourceState).
... and a bunch of others.
PostProcessWinRT:RunReferenceRewriter() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessWinRT.cs:1062)
PostProcessWinRT:Process() (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessWinRT.cs:217)