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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.

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    $\begingroup$ Interesting question! Hmm... let's see, have there ever been other cases of the Chinese government or military spying in the news? Also, slightly related: Is 45 km unusually close to pass the ISS and deploy a microsatellite without warning? $\endgroup$ – uhoh Aug 20 '18 at 0:30
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    $\begingroup$ Also slightly related: ISS - Tiangong 2 minimum safe distance. $\endgroup$ – Cristiano Aug 20 '18 at 10:25
  • $\begingroup$ Not sure about the title... if it's in an orbit similar to ISS, then it's definitely not 'so high'; 400km of ISS is a rather low orbit with a significant atmospheric drag. $\endgroup$ – SF. Aug 20 '18 at 12:16
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    $\begingroup$ I'm referring to the fact that after the big deboost of about 88 km, they did an even bigger reboost (97 km). Would you suggest a better title? $\endgroup$ – Cristiano Aug 20 '18 at 12:46

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