The most obvious answer is that they don’t want to deorbit the station shortly, but why?
Some calculations with the TLEs and SGP4 show the following Tiangong-2 / ISS close encounters:
2018-07-13 09:11:01.384 Dmin= 25.93 km, Vrel= 6.02 km/s 2018-07-30 15:34:39.470 Dmin= 23.16 km, Vrel= 7.30 km/s 2018-08-16 08:06:30.046 Dmin= 31.63 km, Vrel= 8.46 km/s
Since the TLEs age is just a few hours, a reasonable accuracy on Dmin should be better than +/- 2 km.
Does the Tingong-2 have some optical/radar capabilities to observe the ISS?
I’m asking because I’m trying to understand whether the Chinese might be interested in close encounters with the ISS.