TYPO3 Core

When deleting a TS node, if a node with the same index as the parent of the deleted node exists at the same level as the deleted node, it is also deleted.

This will become clearer with an example:

page = PAGE
page {
  10 = COA
  10 {
    10 = TEXT
    10.value = A
    20 = TEXT
    20.value = B
    30 = TEXT
    30.value = C
  }
  20 = COA
  20 {
    10 = TEXT
    10.value = 1
    20 = TEXT
    20.value = 2
    30 = TEXT
    30.value = 3
  }
}
page.10.30 >
page.20.30 >

-- Result: (in FE and in the BE Active Typoscrit module) --
page.10.30 and page.20.30 will be deleted but also page.10.10 and page.20.20

page = PAGE
page {
  10 = COA
  10 {
    20 = TEXT
    20.value = B
  }
  20 = COA
  20 {
    10 = TEXT
    10.value = 1
  }
}

I must have missed something, but I've done a few tests and I can't figure out what $node->removeChildByName($previousIdentifierToken->getValue()); is for
in the function \TYPO3\CMS\Core\TypoScript\AST\AbstractAstBuilder::handleIdentifierUnsetLine