The book’s marginal note explains that when nadmān means nadīm, a companion, its feminine is nadmāna.
In this sense the word is fully declinable: نَدْمَانٌ، نَدْمَانًا، نَدْمَانٍ.
So an adjective’s visible pattern alone does not automatically establish diptote status.
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Diptote Causes — Additional Alif and Nūn and Their Conditions
The book’s marginal note explains that when nadmān means nadīm, a companion, its feminine is nadmāna.
In this sense the word is fully declinable: نَدْمَانٌ، نَدْمَانًا، نَدْمَانٍ.
So an adjective’s visible pattern alone does not automatically establish diptote status.