Why are miaphysite/ old Oriental churches called Orthodox when they are not Orthodox and not related to (Eastern) Orthodoxy?

Well, OP, at least you’re not calling them Monophysites. 🙂

The Greek Wikipedia, and as far as I can tell the Greek Orthodox Church, refers to Oriental Orthodoxy as Pre-Chalcedonian Orthodoxy (Προχαλκηδόνιες Εκκλησίες – Βικιπαίδεια). Of course, a church who thought Chalcedon got it wrong is not going to call itself that.

Orthodoxy – Wikipedia points out the following:

The Homoousian doctrine, which defined Jesus as both God and man with the hypostatic union of the 451 Council of Chalcedon, won out in the Church and was referred to as orthodoxy in most Christian contexts, since this was the viewpoint of the majority. (The minority nontrinitarian Christians object to this terminology).

…

Following the 1054 Great Schism, both the Western and Eastern Churches continued to consider themselves uniquely orthodox and catholic. Over time, the Western Church gradually identified with the “Catholic” label, and people of Western Europe gradually associated the “Orthodox” label with the Eastern Church (in some languages the “Catholic” label is not necessarily identified with the Western Church). This was in note of the fact that both Catholic and Orthodox were in use as ecclesiastical adjectives as early as the 2nd and 4th centuries respectively.

Note also the title of the Pope of the Coptic Orthodox Church of Alexandria:

Pope and Lord Archbishop of the Great City of Alexandria and Patriarch of All Africa on the Holy Orthodox and Apostolic Throne of Saint Mark the Evangelist and Holy Apostle that is, in Egypt, Pentapolis, Libya, Nubia, Sudan, Ethiopia, Eritrea and all Africa.

Orthodox also shows up in the title of Ignatius Aphrem II of Antioch; in Syriac it’s presumably calqued:

English: His Holiness Moran Mor Ignatius Aphrem II, Patriarch of Antioch and All the East and Supreme Head of the Universal Syriac Orthodox Church

Syriac: Qaddišuṯeh ḏ-Moran Mor[y] Iḡnaṭius Afrem Trayono Paṭriarḵo ḏ-Anṭiuḵia waḏ-Kuloh Maḏĕnḥo w-Rišo Gawonoyo ḏ-ʿItto Suryoyto Triṣaṯ Šuḇḥo ḇ-Kuloh Tiḇel

Arabic: Qadāsa Mār ʾIġnāṭīūs ʾAfrām al-Ṯānī Baṭriyark li-ʾAnṭākya wa-Sāʾir al-Mašriq wa-Raʾīs ʾAʿlā lil-Kanīsa al-Suryāniyya al-ʾUrṯūḏaksiyya fī al-ʿĀlam

What this tells me is:

• Orthodox was the name Christians who felt they were not heretics called themselves from the 4th century.
• The Chalcedonian churches called themselves Orthodox. The Miaphysite churches, I’m assuming, would have retorted that they were Orthodox. If the Syriac for Orthodox is a calque, that certainly tells me that Syriac Miaphysites were well aware of the term, and happy to use it for themselves.
• To go by the title of the Coptic Pope and the Syriac Patriarch, they certainly regard their see as Orthodox, and likely have done so for a very long time.
• After the Great Schism, Western Christianity moved away from the term Orthodox, and went with Catholic instead. There was no move away from the term Orthodox in the Miaphysite churches; and Roman Catholic activity in the Middle East would have discouraged them from retaining Catholic.