ARN format is hard coded for assuming the role: 
 
 String roleARN = String.format("arn:aws:iam::%s:role/%s", accountId, this.step.getRole()); 
 (Line 205 of https://github.com/jenkinsci/pipeline-aws-plugin/blob/master/src/main/java/de/taimos/pipeline/aws/WithAWSStep.java) 
 
The above definition works fine for other regions but China since China region is using the "arn:aws-cn:iam" format.