Since Maven 3.0 the relative path of a parent was always verified, an invalid value or missing caches parent made the project fail.
This stopped after 3.5.0, and after a bisect I discovered the behavior changed with:
cfb075ac706b25df630f3671f61f8d8313e0f138 is the first bad commit
Author: Karl Heinz Marbaise <firstname.lastname@example.org>
Date: Tue May 31 21:39:31 2016 +0200
[MNG-6030] ReactorModelCache do not used effectively after maven version 3.0.5 which cause a large memory footprint
o Reintroduced ReactorModelCache reduces the memory footprint.
It looks like the caching introduced an unwanted behavior of Maven. We must decide if we want the relative path to be strict again.