The bundles of a PAR need to share resources between them using import-package and export-package rather than relying on the synthetic context bundle. The synthetic context is provided so that 3rd party libraries *outside* the PAR will get suitable thread context class loading for the PAR.
As described, I would expect each bundle to be able to see only its own resources, which I think matches the behaviour you are finding. So you need to add import-package statements to the bundles that need to see resources in other bundles.
Please try that and see what progress you make.