locallang.xlf merge does not work

[Gernott]

Describe the bug When you add own labels into an xlf file and edit the extension with the extension_builder and save, the own labels are removed. settings.yaml:

  Resources:
    Private:
      Language: merge

To Reproduce Steps to reproduce the behavior:

1. Add one or more labels by hand to locallang.xlf in your extension e.g.:

<trans-unit id="firstname" resname="firstname">
	<source>Firstname</source>
</trans-unit>

2. Set the Language folder to merge in your settings.yaml
3. Open your extension with the extension_builder again and save it
4. The own label is removed

Expected behavior The own label should not be removed, as in earlier versions of the extension_builder.

TYPO3 Version 10.4.23

Extension Builder Version: TER version 10.0.3

[davsalv]

I can confirm the same issue with

TYPO3 Version: 11.5.13

Extension Builder Version: TER version 11.0.3