TYPO3 Core

The example tx_styleguide_inline_usecombination has two buttons:

Button 1 shall create a new record, of the child table.
Button 2 (beside the select field) shall create a relation to an existing record of the child table.

The behavior is this:

Button 1 is creating new records only if there are no records to create a relation. Else a relation is created and the label on the button turns out to be misleading.
Button 2 (beside the select field) is creating a relation to an existing record and if no records are available anymore (all existing ones have already a relation) a new record is created.

Both buttons work similar and depend on the situation if a child record to create a relation is available.
If there exist child records without relation a relation is created, else a new record.
So the behavior of the buttons never depends at all on their intended purpose but on the potential child records.

Comment:
I've an extension that is using the feature with the two buttons and I'd need a core-fix that the extension is running well.

The issue is described also here:
https://github.com/TYPO3/styleguide/issues/331