Do not classify every adjective on an أَفْعَلُ-like shape automatically.
أَحْمَرُ → حَمْرَاءُ: the feminine is not made with round tā, so the taught condition is fulfilled.
أَرْمَلٌ → أَرْمَلَةٌ: the feminine is made with round tā, so this adjective does not qualify through the taught adjective route.
The feminine form supplies the decisive contrast.
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Cause of Ghayr Munsarif: The Pattern of a Verb
Do not classify every adjective on an أَفْعَلُ-like shape automatically.
أَحْمَرُ → حَمْرَاءُ: the feminine is not made with round tā, so the taught condition is fulfilled.
أَرْمَلٌ → أَرْمَلَةٌ: the feminine is made with round tā, so this adjective does not qualify through the taught adjective route.
The feminine form supplies the decisive contrast.