My experience with the great firewall of China
blog.zorinaq.com
blog.zorinaq.com
Where you can see that it does use some tricks (with maybe ML but maybe just hackers adding tricks) to find what you are trying to browse via encryption. Many VPNs work fine with a lot of URLs but will do 100% packet drop on Facebook ; even though I checked for obvious leaks. It is clear something is leaking but it is not obvious.
Yow, I did not know that, but it explains a lot.