Why does آخَرُ lack tanwīn in «طَلْحَةٌ آخَرُ»?

Restoring Declension: Indefinite Names, Al- and Iḍāfah

• Ṭalḥah is a muḍāf
• Ākhar is itself a diptote adjective
• It is a present-tense verb
• It is genitive