That happened due to the China-South Korea trade war following the installation of THAAD in SK in 2016 [0]. Notice how the Chinese OEM spike and Samsung's decline happen following the 2016-17 diplomatic crisis. It was also during this period that Korea Inc began shifting to Vietnam [1][2] and India as a result.

Additonally, that spike for Xiaomi and other Chinese OEMs also happened right when Chinese OEMs expanded their India business in 2015-17 [3][4][5]. On that note, notice how all those Chinese OEM saw sales dropped and then flatlined from 2021 onwards. While the pandemic did play a role, India began lawfare against Chinese companies following the Galwan Crisis in 2021 [6][7][8] with the Indian government de facto forcing Chinese firms to "indianize" [9] - which ironically is similar to how the Chinese government operated in the 2000s and 2010s with Western firms and what the Chinese government leveraged against Korea a decade previously.

[0] - https://www.bloomberg.com/news/articles/2016-08-12/china-sai...

[1] - https://en.yna.co.kr/view/AEN20181122001200320

[2] - https://www.bloomberg.com/news/articles/2018-04-02/s-korea-d...

[3] - https://www.forbes.com/sites/baxiabhishek/2017/09/12/the-ris...

[4] - https://indianexpress.com/article/technology/tech-news-techn...

[5] - https://telecom.economictimes.indiatimes.com/news/oppo-grew-...

[6] - https://www.reuters.com/world/india/india-seizes-725-mln-xia...

[7] - https://www.reuters.com/world/india/india-accuses-chinas-opp...

[8] - https://www.reuters.com/world/india/indias-enforcement-direc...

[9] - https://etplay.com/business/why-chinese-cos-have-been-indian...