Is there any reason, Rain, to think there's a political backstory to this? (Ding Ning is hotpotting with Liu Guoliang? Noooooo....)
Let me first explain the qualification of World Cup. In recent years, World Champion is qualified for World Cup for two years and Asia Cup champion and runner are qualified (others qualifiers are determined by their world ranking and continental Cup result; up to 2 players per association). If CNT wants someone NOT to play in World Cup, they can decline the invitation before the confirmation deadline and the next CNT player will be invited.
For example, as 2015 WTTC champions, DN and ML are qualified for 2015 and 2016 World Cup; as 2017 WTTC champions, they are also qualified for 2017 and 2018 World Cup. In 2015, LSW was the runner-up of 2015 Asia Cup and ZYL the third place; XX the champion and FZD the runner-up. According to the qualification rule, DN & LSW and ML & XX were invited; later CNT withdrew DN and XX, so LSW & ZYL and ML & FZD played. In 2016, LSW was the champion of 2016 Asia Cup and LXX the runner-up; XX the champion and ZJK the runner-up. According to the qualification rule, DN & LSW and ML & XX were invited; DN and ML withdrew first (LXX got retired and no news about ZJK would play), so only LSW and XX would go; later LSW withdrew so no CNT player played in 2016 women World Cup; JM withdrew and FZD was invited for 2016 men World Cup. In 2017, ZYL was the champion of 2017 Asia Cup and LSW the runner-up; LGY the champion and FZD the runner-up. According to the qualification rule, DN & ZYL and ML & LGY were invited; CNT withdrew DN, ZYL & LSW and ML & LGY will play.
Therefore, it is NOT unusual at all that the first two CNT qualifiers for World Cup may withdraw actively or passively; however, DN's case this year is odd because the withdrawal is too sudden after DN is CONFIRMED (CNT can decline her attendance at the first place as the 2015 case) and there is no report of her recent "injury" after winning National Games WS champion in early Sep. Also it does not make much sense to let her play in Swedish Open but not German Open just a few days before if she recovers from the "injury", especially CNT does not limit the number of players for ITTF tours now. DN is the captain of women's team and highly regarded after winning National Games by mainstream media; and she will attend 19th National Congress of CCP. I don't see why she is and shall be affected by LGL or KLH. I would suspect that the management of CNT without head coaches is in a mess now and there is no reasonable planning for a senior player like DN.