ADAL - Bearer token not authenticated (IDX10500: Signature validation failed)

Question

I'm developing a Xamarin Forms app with a .NET WebApi middleware.

On Xamarin layer I'm performing ADAL Login, like this (iOS implementation):

        var authContext = new AuthenticationContext(authority + tenantId);
        if (authContext.TokenCache.ReadItems().Any())
            authContext = new AuthenticationContext(authContext.TokenCache.ReadItems().First().Authority);

        var controller = GetVisibleViewController();
        var uri = new Uri(returnUri);
        var platformParams = new PlatformParameters(controller);
        var authResult = await authContext.AcquireTokenAsync(resource, 
                     clientId, uri, platformParams, UserIdentifier.AnyUser);

Login works, I get the token and I pass it to the WebAPI layer like this:

        httpClient.DefaultRequestHeaders.Authorization =
            new AuthenticationHeaderValue("Bearer",
            authResult.AccessToken);

My WebAPI layer is setup to accept jwt bearer auth:

        app.UseJwtBearerAuthentication(new JwtBearerOptions
        {
            Authority = Configuration["Authentication:AzureAd:AADInstance"] 
                        + Configuration["Authentication:AzureAd:TenantId"],
            Audience = Configuration["Authentication:AzureAd:Audience"]
        });

But, when I make a request to a controller with [Authorize] I get this error:

2017-12-29 11:50:41.134 +00:00 [Information] Failed to validate the token "....".
Microsoft.IdentityModel.Tokens.SecurityTokenInvalidSignatureException: IDX10500: Signature validation failed. No security keys were provided to validate the signature.
   at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.ValidateSignature(String token, TokenValidationParameters validationParameters)
   at System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler.ValidateToken(String token, TokenValidationParameters validationParameters, SecurityToken& validatedToken)
   at Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerHandler.<HandleAuthenticateAsync>d__1.MoveNext()
2017-12-29 11:50:41.149 +00:00 [Information] "Bearer" was not authenticated. Failure message: "IDX10500: Signature validation failed. No security keys were provided to validate the signature."

What is this signature is talking about? Is the problem on client or middleware layer?