Jenkins: Master node doesn't see changed environment variables on slave node

We have a build job that runs on a Jenkins slave node (Windows machine). The job uses environment variables defined in Windows on the slave.

Now, we had to change the values of some of the environment variables on the slave. When we call 'set' on the slave machine it shows the changed values correctly.

But when we start the build job on Jenkins and call 'set' there (Windows Batch Command) then the log still shows the old values of the environment variables! The same wrong values we see when we go to 'Jenkins' > 'Nodes' > '' > 'System Information'.

Do you know what causes the problem and how it can be fixed?

Thank you.