The background of TextArea is not rendered in Fusion style. See the following code and attached screenshot. Without the placeholder text, it wouldn't even be possible to see that there exists a text area. Also, there is no indication whether the text area has the focus. I tested this on Debian and Ubuntu.

import QtQuick
import QtQuick.Controls

ApplicationWindow {
    width: 200
    height: 200
    visible: true

    Column {
        spacing: 20

        TextArea {
            placeholderText: "TextArea"
        }
        TextField {
            placeholderText: "TextField"
        }
    }
}