The translation behavior of IRRE child records on workspaces is wrong on plain new records.
Inline Relational Records editing used the most specific version of a record for references. In case of creating new records on a workspace, a version placeholder record will be generated as well as a specific version to overlay that placeholder. Translations of a record will copy the accordant record and refer to it by using the l18n_parent field ("transOrigPointerField" in TCA). For historic reasons the uid of the placeholder record is used in that case instead of the most specific version - otherwise relations between original and translation are not recognized in several places in TYPO3 (which is actually the real bug, but cannot be resolved easily on the fly for historical reasons).
Thus, the bug fix is to use the uid of the placeholder record in those cases instead and to adopt the form view component of IRRE to recognize those placeholders as translation parents as well.