The textarea for configuring Shell or Batch scripts ToolInstallers is linked to a check method:
https://github.com/jenkinsci/jenkins/blob/39dfa055b0e3bcf3a0fdb1f12c0de9673ab8581c/core/src/main/java/hudson/tools/AbstractCommandInstaller.java#L90

When the script is modified, it gets URL-encoded and sent as parameter of a GET query, which easily fails if the script is a bit verbose (Jenkins gives an error 414 if the URL is >8KB for instance, but the error may be different if using proxy servers).

I see the Pipeline editor textarea uses POST request instead:
https://github.com/jenkinsci/workflow-cps-plugin/blob/c69e3862bf26ac122dbc26345f1bb73b906c6824/src/main/resources/org/jenkinsci/plugins/workflow/editor/workflow-editor.jelly#L16

The linked PR is simply about doing the same for the AbstractCommandInstaller.command textarea.

Note that an alternative would be to get rid of the "doCheckCommand" method, whose only purpose is to check the script is not empty...