The source identifies فِيهِ مُهَانًا in Sūrat al-Furqān as an exception. In «فِيهِ», an unvowelled yāʾ precedes hāʾ, so the ordinary ṣilah condition would not hold; nevertheless, this specific occurrence is given ṣilah.
Do not generalize the exception to every occurrence of فِيهِ.
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Exception: fīhi muhānan
Rules of Hāʾ al-Ḍamīr: Vowels, Ṣilah and Exceptions
The source identifies فِيهِ مُهَانًا in Sūrat al-Furqān as an exception. In «فِيهِ», an unvowelled yāʾ precedes hāʾ, so the ordinary ṣilah condition would not hold; nevertheless, this specific occurrence is given ṣilah.
Do not generalize the exception to every occurrence of فِيهِ.