1- There is no doubt that it’s Mustahab to have the hanak of the turban under the neck/chin.
2- Narrations indicate it’s makrouh not do that especially in prayer and when traveling.
3- Many scholars today, and I have personally seen them, do practice the hanak. Not all of them, but many maraje’ do, especially in prayer.
4- Makrouh can mean less reward. It’s mustahab to wear a turban, and with the hanak the reward is greater. Without it, the reward is less.
So it is a common practice among scholars even today, though I admit it’s less than before. There are no traditions against it. It’s just that over time, turbans began to be wrapped in a firmer way, and opening the hanak makes the turban loose, so possibly to avoid having to keep wrapping it frequently many don’t open the hanak.