The marginal note singles out كَأَيِّنْ. Ordinary tanwīn does not usually display a separate nūn, but here the nūn is written in the muṣḥaf and stopping on it is established in the reading of Ḥafṣ.
Therefore the nūn is not deleted at a stop in this particular word. This exception does not authorize sounding a tanwīn nūn at ordinary stops elsewhere.
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The exceptional written nūn of kaʾayyin
Meeting of Two Sākin Letters — ʿAlā Ḥaddih, Ghayr Ḥaddih and Tanwīn
The marginal note singles out كَأَيِّنْ. Ordinary tanwīn does not usually display a separate nūn, but here the nūn is written in the muṣḥaf and stopping on it is established in the reading of Ḥafṣ.
Therefore the nūn is not deleted at a stop in this particular word. This exception does not authorize sounding a tanwīn nūn at ordinary stops elsewhere.