Answer
From 23R1 onward, use User Group Provisioning with Azure Active Directory rather than trying to assign a Permission Set directly per-user through SCIM — grant the Permission Set to a User Group in IFS, then have SCIM provisioning add newly-provisioned users to that same User Group; this achieves the default-permission-set-on-provisioning outcome indirectly, through group membership, since direct per-user permission-set assignment isn't itself exposed as a SCIM provisioning option.