Wordpress WPAllImport: Own PHP Function if array to string

Question

My JSON import URL has a field, which sometimes contains a single text or an array.

JSON Array:

wp_u_umlaufart  
0   "Personal"
1   "Organisation"

Simple Text:

wp_u_umlaufart  "Organisation"

With the function I tried to check if it is an array, if yes then return this as string and if not then just the text.

My Function:

function my_array_convert( $content ) {
    if (is_array($content)) {
        $converted_arr = implode(",", $content);
        return $converted_arr;
    } else {
        return $content;
    }
}

And this is my custom-field:

Unfortunately, only the texts arrive, an array is not returned.

I changed the function to:

function my_array_convert( $content ) {
    $obj = json_decode(json_encode($content), true);
    if (is_array($obj)) {
        $converted_arr = implode(",", $obj);
        return $converted_arr;
    } else {
        return $content;
    }
}

WPIMPORT does not output this correctly, I have not found an error so far.

But if i put this in my php Sandbox it will work:

<?php

$content = ["Personal","Organisation"];

$obj = json_decode(json_encode($content), true);

echo is_array($obj) ? 'Yes Array' : 'No Array';

if (is_array($obj)) {
   $converted_arr = implode(",", $obj);
   echo $converted_arr;
} else {
   echo $content;
}

?>