Possible Cause
You are using Xporter on a Docker Image that does not have some necessary dependencies, such as the Nashorn dependency (which represents the Javascript Engine), and it is a library that Xporter uses to resolve Javascript expressions. Without it, Xporter can't process "if" conditions and the majority of Mappings that are included in your documents.
Solution
The recommended approach is to use an official image that includes jjs or Nashorn. When changing your Docker image, please follow the instructions of the Advanced Configuration section at this Atlassian Docker page.
To confirm this, you can look for the Nashorn.jar file that should be located at Java_Home>jre>lib.