OWIN Authentication with Azure AD B2C OP: SIgnOut Doesn't Remove Cookie when Called from External iFrame - owin

Question

I have an authentication module in Umbraco using OWIN/OIDC, authenticating against our Azure AD B2C resource. As part of this module, there is a LogOut controller method, which is working correctly.
We are trying to develop single sign out for applications within our Azure tenant. We are still working on having Azure AD B2C call the logout method for each application. In order to test initiation of sign out from other applications, I have set up an iframe in one of our custom applications (also authenticating via Azure AD B2C) that calls the LogOut method in our Umbraco implementation when users sign out from that application. I can see that the LogOut method is being called when the external method opens the iframe, and all of the objects look the same as when the method is called from within Umbraco. However, the user is not logged off of the application. The authentication cookie, which is .AspNet.ApplicationCookie, has SameSite as None, Secure as true and HttpOnly as false, but it is not removed as it is when Umbraco calls the method.
Any tips on how to get the LogOut method to work from the external application would be appreciated.
Here is my configuration:
private void ConfigureAzureB2CAuthentication(object sender, OwinMiddlewareConfiguredEventArgs args) {
//get appbuilder
AppBuilder app = (AppBuilder)args.AppBuilder;
app.SetDefaultSignInAsAuthenticationType(DefaultAuthenticationTypes.ApplicationCookie);
app.UseCookieAuthentication(Current.Factory.GetInstance<FrontEndCookieAuthenticationOptions>(), PipelineStage.Authenticate);
//Set configuration on appbuilder
app.UseOpenIdConnectAuthentication(
new OpenIdConnectAuthenticationOptions {
MetadataAddress = string.Format(
ConfigurationManager.AppSettings["ida:AzureInstance"],
ConfigurationManager.AppSettings["ida:Tenant"],
ConfigurationManager.AppSettings["ida:SignUpSignInPolicyId"]),
ClientId = ConfigurationManager.AppSettings["ida:ClientId"],
RedirectUri = ConfigurationManager.AppSettings["ida:RedirectUri"],
PostLogoutRedirectUri = ConfigurationManager.AppSettings["ida:RedirectUri"],
Notifications = new OpenIdConnectAuthenticationNotifications {
RedirectToIdentityProvider = OnRedirectToIdentityProvider,
AuthorizationCodeReceived = OnAuthorizationCodeReceived,
AuthenticationFailed = OnAuthenticationFailed
},
TokenValidationParameters = new Microsoft.IdentityModel.Tokens.TokenValidationParameters {
NameClaimType = ConfigurationManager.AppSettings["ida:ClaimsLabelEmail"],
ValidateIssuer = false
},
Scope = ConfigurationManager.AppSettings["ida:ScopesOpenIDConnect"],
});
//reafirm backoffice and preview authentication
app.UseUmbracoBackOfficeCookieAuthentication(_umbracoContextAccessor, _runtimeState, _userService, _globalSettings, _securitySection, PipelineStage.Authenticate)
.UseUmbracoBackOfficeExternalCookieAuthentication(_umbracoContextAccessor, _runtimeState, _globalSettings, PipelineStage.Authenticate)
.UseUmbracoPreviewAuthentication(_umbracoContextAccessor, _runtimeState, _globalSettings, _securitySection, PipelineStage.PostAuthenticate);
}
and this is the LogOut method:
public void LogOut(string redirectUrl = "/") {
if (Request.IsAuthenticated) {
RemoveLoggedInMemberAccessToken();
IEnumerable<AuthenticationDescription> authTypes = HttpContext.GetOwinContext().Authentication.GetAuthenticationTypes();
AuthenticationProperties authenticationProperties = new AuthenticationProperties { RedirectUri = redirectUrl };
HttpContext.GetOwinContext().Authentication.SignOut(authenticationProperties, authTypes.Select(t => t.AuthenticationType).ToArray());
}
}