Hi! Thank you for taking the time to report this issue and suggest a fix.

I will look into this today and (hopefully) merge and deploy a fix for this tomorrow.

For now, I'd like to make a reproduction scenario and an automated test to validate the correct behaviour.

Any updates will be posted here.

Create a new directory with composer.json:

{
  "require-dev": {
    "dealerdirect/phpcodesniffer-composer-installer": "0.5"
  },
  "scripts": {
    "post-install-cmd": [
      "echo \"Success\""
    ],
    "post-update-cmd": [
      "echo \"Success\""
    ]
  }
}

Execute composer install and observe "Success".

Change the version number to 0.6. Run composer update twice. All subsequent invocations of composer install or composer update will fail to print "Success". Return to version 0.5. Run composer update twice. All subsequent invocations will succeed.

Thank you for the detailed description! 👍
My test scenarios for this issue had a similar setup, but the details helped speed things up a bit.

The change fixes all the scenarios I created for this issue.
Since they do not break any of the already existing test scenarios, I am merging this.

Thanks again for your help!

The fix for this issue has been release as part of v0.6.2.
Feel free to update your project(s) as needed.