TYPO3 Core

In TYPO3 8.7.1 I have the following TCA:

'date' => [
      'exclude' => true,
      'l10n_mode' => 'exclude',
      'l10n_display' => 'defaultAsReadonly',
      'label' => 'Date',
      'config' => [
        'type' => 'input',
        'eval' => 'date',
        'disableAgeDisplay' => 1,
        'default' => time()
      ],
    ],

The system is set to German as default and English as secondary language.

Now when I edit the record in TYPO3 backend and save the date in German, it is correctly set for the English record as well - BUT when I edit the English record (where I can't set the date, because the field is set to "defaultAsReadonly"), the date for the English record is changed to one day before after/on save. The German Record remains unchanged. The behaviour doesn't seem to be connected to the l10n_display setting; if it is ommited, the field is not displayed, but nevertheless changed on save.