The opening letter «ع» is spoken as «عَيْنْ». In that name, still ya follows fathah, not kasrah. Thus the ya is a soft letter (lin), not an ordinary madd ya. The final nun is still, producing a special case of elongating a soft vowel-glide.
In «كهيعص», the ‘ayn is pronounced by its established full name, not as an arbitrary short syllable. The traditional discussion treats its permissible prolongation with distinctive qualifications, rather than simply placing it under every other three-sound letter-name without distinction.
The decisive point is the preceding vowel: fathah before still ya makes it lin, whereas kasrah before still ya makes it a madd letter. Listening to a correct pronunciation of the letter-name ‘ayn reinforces this distinction.