OIDC doesn't support IDP initiated flows for a reason - they're vulnerable to a class of attacks known as Login CSRF. An attacker can trick a victim into submitting an IdP-initiated authentication response linked to the attacker's account/session into the victim's browser profile on the SP. Think - Eve tricks Bob into signing in to Eve's PayPal account instead of Bob's. Bob, none the wiser, links his bank account. Eve now has access to Bob's funds.
IdPs that need to support an "IdP-initiated" user experience (like a portal or dashboard where users click an app icon) should instead have that icon link to a specific landing page on the SP that safely kicks off a standard, SP-initiated OIDC flow. IdP -> (SP -> IdP -> SP). Look at Okta's "Initiate Login URI" for an example of this.
IdPs that need to support an "IdP-initiated" user experience (like a portal or dashboard where users click an app icon) should instead have that icon link to a specific landing page on the SP that safely kicks off a standard, SP-initiated OIDC flow. IdP -> (SP -> IdP -> SP). Look at Okta's "Initiate Login URI" for an example of this.