MsalServiceException AADSTS50049 when targeting B2C

I just migrated our code to MSAL 2.2.0 and also changed to use the b2clogin.com domain instead of the login.microsoft.com domain we were using before.

I cannot seem to figure out how to construct the correct authority string. Here is what my code currently looks like:

private const string mTenant = "ideliverableb2c.onmicrosoft.com";
private const string mClientId = "d77e...caa6"; // Obfuscated here
private const string mSignInPolicyName = "B2C_1_si";

private PublicClientApplication mClientApplication =
    new PublicClientApplication(mClientId, authority: $"https://ideliverableb2c.b2clogin.com/tfp/{mTenant}/{mSignInPolicyName}")
    {
        RedirectUri = $"msal-{mClientId}://auth",
        ValidateAuthority = false
    };

I have tried various different ways to format the authority string, but no matter what I do I get the same exception when I call mClientApplication.AcquireTokenAsync():

Microsoft.Identity.Client.MsalServiceException: AADSTS50049: Unknown or invalid instance.

Response body:

{
    "error":"invalid_instance",
    "error_description":"AADSTS50049: Unknown or invalid instance.\r\nTrace ID: 6b61d006-7a2a-4b7e-b497-3cd6d9dc3800\r\nCorrelation ID: 78054f61-e0a9-4685-b761-64bc728ff90d\r\nTimestamp: 2018-10-09 19:28:23Z",
    "error_codes":[50049],
    "timestamp":"2018-10-09 19:28:23Z",
    "trace_id":"6b61d006-7a2a-4b7e-b497-3cd6d9dc3800",
    "correlation_id":"78054f61-e0a9-4685-b761-64bc728ff90d"
}

What am I doing wrong here?