The reason behind IPv6 adoption rate dramatical drop in China according to Google measurements?

Google has an IPv6 measurement page that reports that their numbers report on the percentage of users that access Google over IPv6.

According to the report by Jan 2020 0.3% of users in China used IPv6 to access Google

However, looking at this metric in dynamic we see the substantial drop starting from June 2019. enter image description here

I failed to find any solid news that may cause such behavior. I have two hypotheses in mind.

  1. Also as it is a percentage metric, they can adjust their calculation on the total internet penetration rate in China.
  2. Previously open discussions between netizens took place on Google Plus groups. In April 2019, Google shut down Google Plus. Technical discussions continue on Chinese-language blogs, forums, and groups. For obvious reasons, discussions must be hosted outside China, and posters must register under pseudonyms. So probably that caused the shift from Google services but I hardly believe that it may cause such plummet.