Create Pipeline via Azure Powershell and pipelineReturnValue activity is not properly created

I try to run this command in Azure Powershell

Set-AzDataFactoryV2Pipeline -Force -ResourceGroupName MY_DEV_RESOURCE_GRP -DataFactoryName MYADF001 -Name PL_PROCESS_API -DefinitionFile ./PL_PROCESS_API.json

Inside a PL_PROCESS_API.json file is containing this content

...{
            "name": "Activity_Return",
            "type": "SetVariable",
            "dependsOn": [
                {
                    "activity": "Process API data",
                    "dependencyConditions": [
                        "Succeeded"
                    ]
                }
            ],
            "policy": {
                "secureOutput": false,
                "secureInput": false
            },
            "userProperties": [],
            "typeProperties": {
                "variableName": "pipelineReturnValue",
                "value": [
                    {
                        "key": "strCommaSeparatedString",
                        "value": {
                            "type": "Expression",
                            "content": "@activity('Process API data').output.Holiday"
                        }
                    }
                ],
                "setSystemVariable": true
            }
        }...

When I run this command the sections of pipelineReturnValue in type properties in ADF when the command run successfully will be like this

"typeProperties": {
                "variableName": "pipelineReturnValue",
                "value": "[{\"key\":\"strCommaSeparatedString\",\"value\":{\"type\":\"Expression\",\"content\":\"@length(activity('Process API data').output.Holiday)\"}}]"
            }

I would like to know how to fix this?

I have tried the same PowerShell script with below JSON which is similar as your pipeline from my end.

{
    "name": "test2",
    "properties": {
        "activities": [
            {
                "name": "Activity_Return",
                "type": "SetVariable",
                "dependsOn": [
                    {
                        "activity": "Process API data",
                        "dependencyConditions": [
                            "Succeeded"
                        ]
                    }
                ],
                "policy": {
                    "secureOutput": false,
                    "secureInput": false
                },
                "userProperties": [],
                "typeProperties": {
                    "variableName": "pipelineReturnValue",
                    "value": [
                        {
                            "key": "strCommaSeparatedString",
                            "value": {
                                "type": "Expression",
                                "content": "@activity('Process API data').output.data"
                            }
                        }
                    ],
                    "setSystemVariable": true
                }
            },
            {
                "name": "Process API data",
                "type": "WebActivity",
                "dependsOn": [],
                "policy": {
                    "timeout": "0.12:00:00",
                    "retry": 0,
                    "retryIntervalInSeconds": 30,
                    "secureOutput": false,
                    "secureInput": false
                },
                "userProperties": [],
                "typeProperties": {
                    "method": "GET",
                    "url": "https://reqres.in/api/users?page=2"
                }
            }
        ],
        "variables": {
            "var1": {
                "type": "String"
            }
        },
        "annotations": []
    },
    "type": "Microsoft.DataFactory/factories/pipelines"
}

The pipeline got created with correct expression as shown below.

I have tried by changing some code in the pipeline JSON and the only time I got the similar JSON string for the return variable is when I had set the "setSystemVariable" property to false in the typeProperties of the Activity_Return activity.

This is the activity JSON for your reference:

{
                "name": "Activity_Return",
                "type": "SetVariable",
                "dependsOn": [
                    {
                        "activity": "Process API data",
                        "dependencyConditions": [
                            "Succeeded"
                        ]
                    }
                ],
                "policy": {
                    "secureOutput": false,
                    "secureInput": false
                },
                "userProperties": [],
                "typeProperties": {
                    "variableName": "pipelineReturnValue",
                    "value": [
                        {
                            "key": "strCommaSeparatedString",
                            "value": {
                                "type": "Expression",
                                "content": "@activity('Process API data').output.data"
                            }
                        }
                    ],
                    "setSystemVariable": false 
                }
            }

Similar Result:

When the setSystemVariable property is false, the pipeline variable will not be considered as a pipeline return variable and the expression for that will be considered as a string. So, make sure the setSystemVariable property is set to true in the pipeline JSON.