.NET Core JWTBearerAuth returning "Unauthorized"

I'm trying to add auth to my .NET Core 3.1 application, however each attempt to use the returned JWT results in 401 Unauthorized. I've gone through a few steps to debug:

1. I've confirmed that the JWT that my authenticate endpoint returns is valid, using this online validator.
2. I've re-ordered my UseRouting(), UseAuthentication(), UseAuthorization(), and UseEndpoints() to every possible order.
3. I've disabled both audience and issuer validation entirely for the time being.

Can someone identify where I've gone wrong, or at the very least, provide some method of properly debugging so I can track the issue down? Thanks. Code below.

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services
        .AddAuthentication(options => { 
            options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
            options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
        })
        .AddJwtBearer(options =>
        {
            var key = Encoding.UTF8.GetBytes("thisismycustomSecretkeyforauthentication");
            options.SaveToken = true;
            options.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuer = false,
                ValidateAudience = false,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = "issuer",
                ValidAudience = "audience",
                IssuerSigningKey = new SymmetricSecurityKey(key)
            };
        });

    services
        .AddControllers();
        .AddMvc(options => { options.EnableEndpointRouting = false; });
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app
        .UseStaticFiles()
        .UseHsts()
        .UseHttpsRedirection()
        .UseMvc(routes => routes.MapRoute(name: "default", template: "{controller=App}/{action=Index}/{id?}"))
        .UseRouting()
        .UseAuthentication()
        .UseAuthorization()
        .UseEndpoints(endpoints => { endpoints.MapControllers(); });

    if (env.IsDevelopment())
        app.UseSpa(spa => spa.UseProxyToSpaDevelopmentServer("https://localhost:22010"));
}

AuthController.cs

[ApiController]
[Authorize]
[Route("[controller]")]
public class AuthController : Controller
{
    [AllowAnonymous]
    [HttpPost("authenticate")]
    public async Task<IActionResult> Authenticate(AuthenticationRequest request)
    {
        if (request.Username != "test" || request.Password != "admin")
            return Unauthorized();

        var tokenHandler = new JwtSecurityTokenHandler();
        var tokenKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("thisismycustomSecretkeyforauthentication"));
        var token = new JwtSecurityToken
        (
            issuer: null,
            audience: null,
            claims: new Claim[] { new Claim(ClaimTypes.Name, request.Username) },
            expires: DateTime.Now.AddDays(30),
            signingCredentials: new SigningCredentials(tokenKey, SecurityAlgorithms.HmacSha256)
        );
        var tokenResponse = tokenHandler.WriteToken(token);

        return Ok(tokenResponse);
    }

    [HttpGet]
    public IActionResult Do()
    {
        return Ok("Done!");
    }
}