TYPO3 Core

We have an extension that creates a custom field for the Extension Manager:

// ext_conf_template.txt

# cat=Basic; type=user[Vendor\Extension\Helpers\MyCustomHelper->textfield]; label=My Label
myVarName = 

The MyCustomHelper class looks like this (just an example, of course senseless as standard "inputs" exist in the Core ;)

<?php

namespace My\Extension\Helpers;

class MyCustomHelper {

    public function textfield( $conf = [] ) 
    {
        return "<input name=\"{$conf['fieldName']}\" class=\"form-control\" value=\"{$conf['fieldValue']}\"></input>";
    }

}

- The field gets rendered correctly, but $conf['fieldValue'] is always empty / NULL.
- If you fill out the field, it gets saved correctly in the LocalConfiguration.php / settings.php - but it is not passed back to the method when the custom form in the Extension Configuration Manager is rendered.
- The saved value can also be accessed using GeneralUtility::makeInstance(ExtensionConfiguration::class)->get('my_extension_key');.

Possible starting point:
- Value exists and is correct in EXT:cms-install/Classes/Controller/SettingsController.php
- Value does NOT exists in EXT:cms-core/Classes/TypoScript/AST/Visitor/AstConstantCommentVisitor.php->parseNodeComment()