Unable to set check source for passive service via API

Hello folks,

I want to process service checks results into passive service via API. There is no problem in processing service state and output, but I am unable to set check_source. Last service check check_source is still set to command_endpoint instead of string I am trying to pass in process-check-result API command altogether with output and state.

My POST Data for process-check-result:
"type": "Service", "filter": "host.name==\"cn201.<XXX>.cz\" && service.name==\"offliner-receiver\"", "exit_status": 2, "plugin_output": "Critical TEST", "check_source": "reporter.example.com", "pretty": true
Please note check_source attribute set to reporter.example.com

API response:
"status": "Successfully processed check result for object 'cn201.<XXX>.cz!offliner-receiver'."

Then I ask for service check results using this GET Data:
type": "service", "filter": "match(\"offliner-receiver\",service.name)", "pretty": true

API response is:

            "attrs": {
                "__name": "cn201.<XXX>.cz!offliner-receiver",
                "acknowledgement": 0.0,
                "acknowledgement_expiry": 0.0,
                "action_url": "",
                "active": true,
                "check_attempt": 1.0,
                "check_command": "passive",
                "check_interval": 60.0,
                "check_period": "",
                "check_timeout": null,
                "command_endpoint": "",
                "display_name": "TEST Offliner Receiver",
                "downtime_depth": 0.0,
                "enable_active_checks": false,
                "enable_event_handler": true,
                "enable_flapping": false,
                "enable_notifications": true,
                "enable_passive_checks": true,
                "enable_perfdata": true,
                "event_command": "eh-offliner",
                "flapping": false,
                "flapping_current": 0.0,
                "flapping_last_change": 0.0,
                "flapping_threshold": 0.0,
                "flapping_threshold_high": 30.0,
                "flapping_threshold_low": 25.0,
                "force_next_check": false,
                "force_next_notification": false,
                "groups": [],
                "ha_mode": 0.0,
                "handled": false,
                "host_name": "cn201.<XXX>.cz",
                "icon_image": "",
                "icon_image_alt": "",
                "last_check": 1585226451.958697,
                "last_check_result": {
                    "active": false,
                    ==> "check_source": "mgmt1.<XXX>.cz", <==
                    "command": null,
                    "execution_end": 1585226451.958697,
                    "execution_start": 1585226451.958697,
                    "exit_status": 0.0,
                    "output": "Critical TEST",
                    "performance_data": null,
                    "schedule_end": 1585226451.958697,
                    "schedule_start": 1585226451.958697,
                    "state": 2.0,
                    "ttl": 0.0,
                    "type": "CheckResult",
                    "vars_after": {
                        "attempt": 1.0,
                        "reachable": true,
                        "state": 2.0,
                        "state_type": 1.0
                    },
                    "vars_before": {
                        "attempt": 1.0,
                        "reachable": true,
                        "state": 2.0,
                        "state_type": 1.0
                    }
                },
                "last_hard_state": 2.0,
                "last_hard_state_change": 1585226451.958697,
                "last_reachable": true,
                "last_state": 2.0,
                "last_state_change": 1585223508.502978,
                "last_state_critical": 1585226451.958702,
                "last_state_ok": 1585206948.281747,
                "last_state_type": 1.0,
                "last_state_unknown": 0.0,
                "last_state_unreachable": 0.0,
                "last_state_warning": 0.0,
                "max_check_attempts": 5.0,
                "name": "offliner-receiver",
                "next_check": 1585226511.958711,
                "notes": "",
                "notes_url": "",
                "original_attributes": null,
                "package": "_etc",
                "paused": false,
                "previous_state_change": 1585223508.502978,
                "problem": true,
                "retry_interval": 30.0,
                "severity": 136.0,
                "source_location": {
                    "first_column": 1.0,
                    "first_line": 412.0,
                    "last_column": 33.0,
                    "last_line": 412.0,
                    "path": "/etc/icinga2/zones.d/<XXX>/services.conf"
                },
                "state": 2.0,
                "state_type": 1.0,
                "templates": [
                    "offliner-receiver",
                    "generic-service"
                ],
                "type": "Service",
                "vars": {
                    "dummy_state": 0.0
                },
                "version": 0.0,
                "volatile": true,
                "zone": "barbora"
            },
            "joins": {},
            "meta": {},
            "name": "cn201.<XXX>.cz!offliner-receiver",
            "type": "Service"

Please note check_source set to command endpoint mgmt1.<XXX>.cz instead of reporter.example.com

Just for sure, this is my passive service config:

Object 'cn201.barbora.it4i.cz!offliner-receiver' of type 'Service':
  * __name = "cn201.<XXX>.cz!offliner-receiver"
  * action_url = ""
  * check_command = "passive"
  * check_interval = 60
  * check_period = ""
  * check_timeout = null
  * command_endpoint = ""
  * display_name = "TEST Offliner Receiver"
  * enable_active_checks = false
  * enable_event_handler = true
  * enable_flapping = false
  * enable_notifications = true
  * enable_passive_checks = true
  * enable_perfdata = true
  * event_command = "eh-offliner"
  * flapping_threshold = 0
  * flapping_threshold_high = 30
  * flapping_threshold_low = 25
  * groups = [ ]
  * host_name = "cn201.<XXX>.cz"
  * icon_image = ""
  * icon_image_alt = ""
  * max_check_attempts = 5
  * name = "offliner-receiver"
  * notes = ""
  * notes_url = ""
  * package = "_etc"
  * retry_interval = 30
  * source_location
    * first_column = 1
    * first_line = 412
    * last_column = 33
    * last_line = 412
    * path = "/etc/icinga2/zones.d/<XXX>/services.conf"
  * templates = [ "offliner-receiver", "generic-service" ]
  * type = "Service"
  * vars
    * dummy_state = 0
  * volatile = true
  * zone = "<XXX>"

Is there anything wrong with my process-check-result Data?

Aim of this is to have information who processed passive check results for that service because there will be many hosts/sources who will be posting passive check results in my environment and I need to differentiate between them (event command will parse check_source, exit state and output).

Thank you very much for any hints.