TYPO3 Core

When configuring a TCA type "slug", using a "field" in "generatorOptipons" with readOnly=true set in TCA, this field is not used/skipped in the slug generation process.
E.g. this TCA

'title' => [
     'exclude' => true,
     'label' => 'LLL:EXT:xyz/Resources/Private/Language/locallang_db.xlf:tx_xyz_domain_model_offer.title',
     'config' => [
         'type' => 'input',
         'size' => 30,
         'eval' => 'trim,required'
     ],
],
'webcode' => [
   'exclude' => true,
   'label' => 'LLL:EXT:xyz/Resources/Private/Language/locallang_db.xlf:tx_xyz_domain_model_offer.webcode',
   'config' => [
       'type' => 'input',
       'readOnly' => true,
       'size' => 4,
       'eval' => 'unique,int',
       'range' => [
           'lower' => 1000,
           'upper' => 9999
       ]
   ],
],
'slug' => [
   'exclude' => true,
   'label' => 'LLL:EXT:xyz/Resources/Private/Language/locallang_db.xlf:tx_xyz_domain_model_offer.slug',
   'config' => [
       'type' => 'slug',
       'fallbackCharacter' => '-',
       'size' => 50,
       'eval' => 'uniqueInSite',
       'generatorOptions' => [
           'fields' => ['title', 'webcode'],
           'fieldSeparator' => '-',
           'prependSlash' => true,
           'prefixParentPageSlug' => true,
           'replacements' => [
               '/' => '',
           ],
       ]
   ],
],

The title field is used correctly, the webcode field is skipped. When I remove the readonly flag on webcode, the slug is generated correctly.