B2B users cannot sign in to Tenant using v2.0 endpoint & MSAL Auth flow

If you pass the specific tenant value in the authority, then

Only users with a work or school account from a specific Azure AD tenant can sign in to the application. Either the friendly domain name of the Azure AD tenant or the tenant's GUID identifier can be used.

That's means the Microsoft Account is not supported in this scenario. Refer here for the Microsoft Account and Work or school accounts. And in this scenario, if you new a user user from other tenant, it should also works.

You can refer the document for tenant from link below:

Fetch the OpenID Connect metadata document

I know this is an old thread but just in case anyone stumbles upon it, here is a solution: In cases of Personal guest accounts, use Credential Grant Flow (Get access without a user). To do that, you would first need to grant appropriate permission (of Type Application) for the API you wanted to use on behalf of the signing user. This would let you acquire access token with the application's identity itself rather than the signed in user. Next get token like this (in this sample, I'm getting access token for Graph API):

public async Task<string> GetAccessToken()
{
    using (HttpClient httpClient = new HttpClient())
    {
        string token = "";
        try
        {
            httpClient.BaseAddress = new Uri($"https://login.microsoftonline.com/{tenantId}");
            httpClient.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/x-www-form-urlencoded; charset=utf-8");
            HttpRequestMessage request = new HttpRequestMessage();
            List<KeyValuePair<string, string>> body = new List<KeyValuePair<string, string>>
            {
               new KeyValuePair<string, string>("client_id", clientId),
               new KeyValuePair<string, string>("scope", "https://graph.microsoft.com/.default"),
               new KeyValuePair<string, string>("client_secret", appSecret),
               new KeyValuePair<string, string>("grant_type", "client_credentials")
            };
            request.Method = HttpMethod.Post;
            request.RequestUri = new Uri($"{httpClient.BaseAddress}/oauth2/v2.0/token");
            request.Content = new FormUrlEncodedContent(body);
            var response = await httpClient.SendAsync(request);
            var content = await response.Content.ReadAsAsync<dynamic>();
            token = content.access_token;
        }
        catch (Exception e)
        {
        }
        return token;
    }
}

Tip: If your goal is also Graph API, don't try to get logged in user info by using the /me endpoint in this case. Since the token was generated using the application identity rather than the signed in user, /me would be the application not the logged in user. What you want to do is: retrieve logged in user id from the Claim (Type: http://schemas.microsoft.com/identity/claims/objectidentifier) and use the /user/{userid} endpoint.

I found: for personal accounts (Get access without a user) in the body of the request you must to use grant_type = 'client_credentials' and for corporate accounts to use grant_type = 'authorization_code'