The affix ñya comes without changing sense, after the words 1. anantā 2. āvasatha 3. itiha and 4. bheṣajā,
Thus आनन्त्यम् = अनन्तः, आवसथ्यम् = आवसथः 'dwelling place, a sanctuary for sacred fire'. ऐतिह्यम् = इति ह 'traditional account', the affix here is added to a word which is not a pratipadika. The word इति ह means tradition, as इति ह स्मोपाध्यायाः कथयन्ति ॥ भैषज्यम् = भैषजम् ॥
It is optional to add this affix, owing to the general optional nature of all taddhita affixes. See 4.1.82.,