Fresh off the press is a new Knowledge Base article at HCL that describes an event where a user authenticating via SAML is not being sent to the originally requested URL after authentication, but is sent to the home URL for the site. This affects new or upgraded Domino 12.0.2 servers.
The cause for this is the new default setting for samesite=strict that is added to the relay state cookie. The notes.ini setting DOMINO_RELAY_COOKIE_SAMESITE=0 will resolve this issue after restarting Domino.
Full article: After authenticating with SAML, the user is not returned to originating URL