The Muṇḍaka-Upaniṣad shows many metrical deviations. In 1924, Johannes Hertel made a critical edition of the Upaniṣad only on the basis of the metrics, but without investigating manuscripts. His text has been used uncritically by some scholars until today, although its reliability is questionable.
Within my doctoral dissertation, I edited a text of the Muṇḍaka-Upaniṣad, which is based on the study of manuscripts and prints. Language, meter, and philosophical ideas have also been analyzed for this edition.
This paper discusses the reason behind the metrical problems in the text. One result of the study shows that metrical deviations sometimes occur when the Upaniṣad quotes some words or passages from other texts including the Āpastamba-Śrautasūtra of the Black Yajurveda and Paippalāda-Saṃhitā of the Atharvaveda. This demonstrates that the preceding texts had a great influence on the composition and development of the Upaniṣad.