Try changing the target framework ordering so that .net 4x is first:

<TargetFrameworks>net472;netstandard2.1</TargetFrameworks>

Also for projects that reference multi-targeted project, try referencing like this. (From: https://stackoverflow.com/a/53388855)

<ProjectReference Include="..\yourMultiTargetedProject.csproj" >
  <SetTargetFramework>TargetFramework=netstandard2.1</SetTargetFramework>