Lhasa, China 144 hours visa free transit restrictions

I was traveling from KTM/Kathmandu, Nepal to CTU/Chengdu, China with a stop in LXA/Lhasa, China. They did not allow me to fly because they said I needed a visa to go to Lhasa. Lhasa was only a stop (I would not leave the airport) then onto a 23 hr 55 min layover in Chengdu before returning to the US. Does anyone know if this was correct? I lost that leg of the flight (non-refundable) and had to purchase another ticket for $900. Everything I had read prior to my trip said I would have no problems and need no visa due to the 144 hr visa free transit. Is LXA/Lhasa an exception? Please advise!