Simple example:

node('master') {
  try {
    timeout(time: 5, unit: 'SECONDS') {
      try {
        sleep(30)
      }
      catch (err) {
        echo "For some reason, this catch is activated. Error: ${err.getMessage()}"
      }
      echo "This should not be executed, but it is!"
    }
  }
  catch (err) {
    echo "I expect that this catch will be activated. Error: ${err.getMessage()}"
  }
}

I expect that when a timeout is reached, the exception will be caught by the try-catch that surrounds the "timeout" step.
But actually, it is caught by the most inner try-catch.
My goal here is to set timeout to set of actions and if the timeout is reached, I want them all to stop. I also want to have more try-catch inside for handling other issues.
But the actual behavior is that when the timeout is reached, the most internal catch handles it instead of the try-catch that surrounds the timeout step.

This issue is similar to JENKINS-39266. I do not understand why it was closed.

By the way, the 

err.getMessage()

prints "null", which is also a problem.