The PRC is threatening to detain US Citizens in China in retaliation for the US detention of Chinese scholars. 

This was foreseeable, particularly given that the US prosecutions appear in some cases to be largely pretextual and politically motivated:

Chinese officials have told the Trump administration that security officers in China might detain American citizens if the Justice Department proceeds with prosecutions of arrested scholars who are members of the Chinese military, American officials said.

The Chinese officials conveyed the messages starting this summer, when the Justice Department intensified efforts to arrest and charge the scholars, mainly with providing false information on their visa applications, the American officials said. U.S. law enforcement officials say at least five Chinese scholars who have been arrested in recent months did not disclose their military affiliations on visa applications and might have been trying to conduct industrial espionage in research centers.

American officials said they thought the Chinese officials were serious about the threats. The State Department has reiterated travel warnings as a result, they said. Western officials and human rights advocates have said for years that the Chinese police and other security agencies engage in arbitrary detentions.

This is not a surprise.