TYPO3 Core

After adding a childelement at the default-language to a already translated element and synchronize it in the translated parent element, an error appears. But it seems that all works fine.

This is easy to reproduce in 2 ways:

1) with an own extension - see attached:

• install a TYPO3 8.7 project
• add a website-language
• create a page and translate it
• install the extension mytest (see attached)
• add a 'parenttop' entry in the list module and add a 'child'
• translate the whole element (all works fine until here)
• now add a child to the exiting parenttop element in the default language. save and close
• edit the translated parenttop element and click the button 'Synchronize with original language'
• the attached errormessage appears.

2) with Mask:

• install a TYPO3 8.7 project
• add a website-language
• create a page and translate it
• install the extension mask
• create a content element with a repeating field and a string filed inside this
• mark all checkboxes in the repeating fields 'Localization' tab, except of 'Show removed localization records'
• create a contentelement in the page module of this type and translate it (works fine)
• edit the default-language element and add another child. close and save
• edit the translation and click the button 'Synchronize with original language'
• the attached errormessage appears.

Perhaps this is related to: https://forge.typo3.org/issues/31812
There is another problem, but it seems very similar.