We use Federated Authentication in Sitecore 9.1 in order to allow a user to login to the extranet domain through an external provider (Azure AD B2C).
When configuring the sign out part, we followed the documentation found on https://doc.sitecore.com/developers/91/sitecore-experience-management/en/understanding-sitecore-authentication-behavior-changes.html#UUID-153eb49e-9b9c-c95e-1e0c-c368778c900d_section-idm45851235771136.
When the user tries to sign out, he/she will end up in a navigation loop.
The logout is initiated by clicking a logout button, which calls
Sitecore appears to initiate a local logoff and then sets the
sc_externalLogout cookie. This will cause a redirect to the signout url of the external provider. The user is signed out on the external provider and redirected back the post logout redirect URI. Sitecore now sends the user again to the signout url of the external provider, causing a navigation loop.