I have been experiencing something similar for my [app][1] too. However after having done some research, I think I understand this situation enough for this to not be a concern to me right now. Again this is purely based on my understanding of the situation.

 1. iTunesConnect App Analytics is ["OPT-IN"][2]: So the users have to agree to sending data to Apple for these things to show up in the App Analytics in iTunesConnect. 
 2. Analytics software: Now China (中国) has this thing they call the 'Great Firewall' and what it does is change the destination of the data being transmitted to servers outside of China. From what I understand Data for Chinese （中国人） users must be stored within China i.e. the server where the data for Chinese users is stored must be physically within China. I believe Google servers are not, hence it's blocked. Google owns Firebase now, ergo that's blocked too.
 3. My case: Now for my app the downloads have increased overall, with over 60% of those coming from China and we are seeing some data (from China, it's not like we get 0 data from China. We see some sessions and why is that the case? Now after some digging around I found out that the way the Great Firewall(Google/Bing it) works is not 100%. The process of changing the destination doesn't always work i.e. it does not always change the destination and therefore some of it goes through and combine that with some tech savvy users using VPN in China. However for my case, it's unlikely that the users of my app are using VPN.




 [1]: https://itunes.apple.com/au/app/my-day-to-do-lite-like-pen-paper-day-planner/id1066820078?mt=8
 [2]: https://forums.developer.apple.com/thread/25491