Create a bug by call rest api, but the post function does not work

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1

I create a custom field called “Bug Description” in create issue screen. I can create set the default value for bug template. And I add a post function to copy the text of “Bug Description” to “Description”. It works fine to create a bug manually.
But I try to create issue by rest api. I can create the issue successfully, but the value of Description is empty. The value of Bug Description is correct. So guess the post function does not work.

This my payload:
{
“update”: {
},

"fields": {
    "summary": "autotest3",
    "issuetype": {
        "name": "Bug"
    },
    "project": {
        "id": "11107"
    },
    "customfield_11278": { 
        "type": "doc",
        "version": 1,
        "content": [
            {
                "type": "paragraph",
                "content": [
                    {
                        "text": "auto111",
                        "type": "text"
                    }
                ]
            }
        ]
    },
    "reporter": {
        "accountId": "60c0e116a01e11006ae66369"
    },
    "priority": {
        "name": "Medium"
    },
    "customfield_11261": {
        "id": "10260" 
    },
    "customfield_11249": {
        "id": "10240" 
    }
}

}