QGIS » QGIS Application

This seems to be an enduring problem or not well documented in Qgis that I can ever remember but it seems to have been working properly in earlier masters and stopped working again in later ones.
I have found almost consistently except for this rare period when it actually worked that every time I set up a new installation of Qgis and load the styles into Style Manager that my styles which have a custom SVG symbol have the symbols missing and I have to every single time go through and add these symbols back to the styles manually.
With a style file like the attached loading it in master 313ec55 will find all the custom SVGs and load them straight off from the xml file, the later masters e.g. the current one will not.
Just to make it even more complex I have some layers with rule based styles, these will not remember the custom SVGs so every time I open a project for the first time after installing Qgis I have to go through and set all these by hand as well.
Given there is a path stored to the SVG file inside the XML file for each style it can't be that hard surely?