Here's the Maven JAR plugin issue tracking this:
https://issues.apache.org/jira/browse/MJAR-275

When creating a modular JAR, the jar tool will write the module-info.class with the current time as the timestamp. We should look at changing this so that it uses the timestamp of the module-info.class on the file system, even for cases where the ModulePackages attribute is added to the module-info.