Please enable the modern UI feature in China.

As I know, there is a geo restriction on Adobe cloud services in China, so PDF services are not available to any users in China.
The Modern UI employs PDF services and was built to use Acrobat AI services, which do not work owing to geo constraints.
As a result, when you sign in to Acrobat after a while, it reverts to the OLD UI, and all UI switching choices are unavailable.