Unable to register a custom macro in Kentico 13

Question

I'm on the latest hotfix (v13.0.151).

I followed the documentation to add a custom macro method but it doesn't register. Below is what I did:

In a separate assembly (which has a dummy class annotated with [assembly: AssemblyDiscoverable]) I have my macro methods:

//[assembly: RegisterExtension(typeof(SOSMacroMethods), typeof(SOSMacroNamespace))] //tried with and without
public class SOSMacroMethods : MacroMethodContainer
{
    [MacroMethod(typeof(string), "test method", 0)]
    public static object MyTestMethod(EvaluationContext context, params object[] parameters)
    {
        return "yayyy registered";
    }
}

And the module and namespaces as so:

[assembly: RegisterModule(typeof(SOSMacroModule))]

public class SOSMacroModule : Module
{
    public SOSMacroModule()
        : base("SOSMacros")
    {
    }

    protected override void OnInit()
    {
        base.OnInit();

        MacroContext.GlobalResolver.SetNamedSourceData("SOS", SOSMacroNamespace.Instance);
    }
}

[Extension(typeof(SOSMacroMethods))]
public class SOSMacroNamespace : MacroNamespace<SOSMacroNamespace>
{

}

I then compiled this and added it to my admin solution as an existing project and added a reference to it too. I then rebuilt the admin solution.

When I go to the admin interface I don't get it in the macro editor and when I try to debug it I get the error log message:

CMS.MacroEngine.EvaluationException: Exception occurred while evaluation of the expression ' <snipped for brevity>': Method 'MyTestMethod' not found, please check the macro or the method declaration.

I get the same issue when trying to resolve the macro on my live site too. Edit: working on live site now though I've made no changes

This works in my K12 Portal solution and from what I can gather the only changes are sharing the macros between presentation and admin in K13.

Is there something I'm missing?