Nev,
“Only with the name Yaakov. No idea why so many of us have that minhag.”
That proves my above point that the Ayin as in “Yankev” is actually the true pronunciation, (as in the Gemara megilah,) with the years it got lost, but it still stuck to the name “Yankev”.
BTW – The Brisker Rav ZTL said Krias Shma with the Ayin.