Feeling kinda stupid as I found the explanation on the PDF named What Are Relocated Items? under the Relocated Items folder. The relevant part for the clarification is:

Configuration files with the suffix "system_default" were edited or customized, but the changes were allowed to remain installed. The system_default version of the file is provided to demonstrate what the Apple supplied version of this file would look like. It is recommended that you compare the two and evaluate whether you wish to integrate any changes Apple may have made to the default version.

As I modified this file, looks like Apple will always provide the default version of this file so I can manually compare it with my modified version and merge any relevant changes. So no way to avoid this behaviour but finally I understand why this happens after each update.

Feeling kinda stupid as I found the explanation on the PDF named What Are Relocated Items? under the Relocated Items folder. The relevant part for the clarification is:

Configuration files with the suffix "system_default" were edited or customized, but the changes were allowed to remain installed. The system_default version of the file is provided to demonstrate what the Apple supplied version of this file would look like. It is recommended that you compare the two and evaluate whether you wish to integrate any changes Apple may have made to the default version.

As I modified this file, looks like Apple will always provide the default version of this file so I can manually compare it with my modified version and merge any relevant changes. So no way to avoid this behavior but finally I understand why this happens after each update.