たまに仕様が変わって動かなくComposerですが、1か月ほど前にまた仕様変更があったようです。

Before, running install without a lock file did an install that was almost an update, but still ran the pre/post-install-cmd scripts. Now we normalized it to actually run an update and fire pre/post-update-cmd, because that's what it is doing. If you have a lock file though it will run install as before.

ということで、lockファイルなしのinstallではpre/post-install-cmdは動作しなくなりました。pre/post-update-cmdを使う必要があります。

参考

• https://github.com/composer/composer/issues/5066#issuecomment-197006540