[Security Solution] - Problem using RelayState in IdP-Initiated Flow

[medina325]

Describe the bug:
As I described here I set up Elastic/Kibana Relying Party (RP) as a SAML Service Provider (SP) and a custom SAML Identity Provider (IdP). Both IdP and SP flows work fine, however I need to redirect the user to specific dashboards in Kibana, from my IdP.
By reading this and this I thought I could do that just by adding a RelayState to my "IdP initiated SAML Response" with the appropriate deeplink to one of my Kibana's dashboards.
What I get instead is Kibana making a GET request to it's ACS url and appending the RelayState's deeplink in the end, after my IdP gets redirected to {kibana_url}/api/security/saml/call.

Kibana/Elasticsearch Stack version:
7.15

Server OS version:
Windows 10

Browser and Browser OS versions:
Firefox, Edge, Chrome

Elastic Endpoint version:
I don't know.

Original install method (e.g. download page, yum, from source, etc.):
Download page.

Functional Area (e.g. Endpoint management, timelines, resolver, etc.):
I don't know.

Steps to reproduce:

1. Set up Elastic and Kibana Relying Party as SAML Service Provider (don't forget to set xpack.security.authc.providers.saml.<provider_name>.useRelayStateDeepLink to true)
2. Set up a custom IdP that supports the use of the RelayState parameter (I used this package since my IdP is a Laravel app).
3. Initiate an IdP initiated flow passing the RelayState parameter with the SAML Response.

Current behavior:
After IdP initiated flow with RelayState, Kibana tries to make a GET request do it's ACS url with RelayState's deeplink appended to it.

Expected behavior:
Kibana redirects to deeplink.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context (logs, chat logs, magical formulas, etc.):
I'm quite new to SAML, did I understand the purpose of RelayState wrong? This is the definition I read:

Any help will be highly appreciated

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[medina325]

The problem was solved here