PlatformUtil.java looks for javafx.platform.properties in two places:

lib/ext in the FX runtime directory
lib/ in the JRE directory

The file is to be found in lib/javafx.platform.properties in the FX runtime directory

If we run as an overlay on the JRE this is fine, because the JRE directory is identical to the FX runtime directory. This is the case that SQE test and this is how we deliver the product.
However, in development we usually want the FX runtime directory to be separate from the JRE directory. So we need PlatformUtil to look for lib/javafx.platform.properties in the FX runtime directory.