I had a terrible experience of not knowing that there was a visa requirement for flying it in different airlines from Nepal to Hongkong and Hongkong to U.S. I have not found an authentic source but I wasn't able to board the plane and had to cancel and buy new tickets.
I was flying from Cathy Dragon and switching to United Airlines at Hongkong.

Any thought feedback is welcome if this is not a true case.

CathayDragon is hongkong based and has visa waiver agreement for Nepalese. Connection has to within Cathay and dragon. Other airlines dont, so you need a visa.

I had the similar experience- Was denied boarding from TIA- dragon/United was my flight. Had to get another ticket on a different day- as it was too expensive for that flight!

They left some of my relatives in Singapore flying from Kathmandu saying that they had no transit visa in Honkong. None talked about the need of the visa while in Kathmandu. Suddenly, they say that you cannot fly at the time to board another plane that had transit in Honkong. They needed to buy much more expensive another flight ticket to get to the USA. All their family and travelers were in a very difficult situation going thru all the hassles .