There are at least two ways to fix this issue in the .csproj (you can use one or the other or both):

1. add this in <PropertyGroup> XML element

   <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   

2. add an explicit package reference to the C#/WinRT package (current version is 2.2.0) to an <ItemGroup> XML element

   <PackageReference Include="Microsoft.Windows.CsWinRT" Version="2.2.0" />
   

I'm not sure why you don't get a compilation warning because AOT compilation usually requires unsafe blocks to be enabled. It may be a bug, seems similar to this: https://github.com/microsoft/CsWinRT/issues/1891