TYPO3 Core

I might have found it:

sysext/backend/Classes/Form/FormDataProvider/TcaSelectTreeItems.php:

protected function prepareAdditionalItems(array $itemArray, array $selectedNodes)
    {
        $additionalItems = [];

        foreach ($itemArray as $item) {
            if ($item[1] === '--div--') {
                continue;
            }

            $additionalItems[] = [
                'uid' => $item[1],
                'text' => $item[0],
                'selectable' => true,
                'leaf' => true,
                'checked' => in_array($item[1], $selectedNodes),
                'icon' => $item[3]
            ];
        }

        return $additionalItems;
    }

protected function prepareAdditionalItems(array $itemArray, array $selectedNodes)
    {
        $additionalItems = [];

        foreach ($itemArray as $item) {
            if ($item[1] === '--div--') {
                continue;
            }

            $additionalItems[] = [
                'uid' => $item[1],
                'text' => \TYPO3\CMS\Extbase\Utility\LocalizationUtility::translate($item[0], ''),
                'selectable' => true,
                'leaf' => true,
                'checked' => in_array($item[1], $selectedNodes),
                'icon' => $item[3]
            ];
        }

        return $additionalItems;
    }

I've checked the presence of the reported code on both 8.7.9 and latest master; I've not found the function prepareAdditionalItems and the file sysext/backend/Classes/Form/FormDataProvider/TcaSelectTreeItems.php now reads (line 129 and followings):

[...]
// And translate any labels from the static list
$staticItems = $this->translateLabels($result, $staticItems, $table, $fieldName);
// Now compile the target items using the same array structure as the "dynamic" list below
foreach ($staticItems as $item) {
    if ($item[1] === '--div--') {
        // Skip divs that may occur here for whatever reason
        continue;
    }
    $finalItems[] = [
        'identifier' => $item[1],
        'name' => $item[0],
        'icon' => $item[2] ?? '',
        'iconOverlay' => '',
        'depth' => 0,
        'hasChildren' => false,
        'selectable' => true,
        'checked' => in_array($item[1], $result['databaseRow'][$fieldName]),
    ];
}
[...]

Maybe it has been solved in the meanwhile?