It fails because the .NET assembly loader tries to find netstandard2.0 dll, the shim dll which redirects code to the real implementations and this fails. So the type is simply returned as 'null' and thus ignored.
I can't find any evidence this should work as well, so I have no idea how to make it work. We could bind to a resolve event to resolve the missing assemblies, but where from?
So this isn't supported, the enum type dll has to be a .net full dll the moment. The best way to do this, is by using multi-targeting.
In your csproj file you have:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
</PropertyGroup>
...
change this to: (pay attention to the changed element name!)
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFrameworks>netstandard2.0;net452</TargetFrameworks>
</PropertyGroup>
...
You'll now get 2 dlls, for the enum projects one for netstandard2.0 and one for net452. Use the latter in your typeimports. This is low friction, you'll reference the netstandard2.0 one elsewhere, and it doesn't require any extra code.