We found the issue and came to a resolution on this. Basically our issue is this https://firstname.lastname@example.org/msg129314.html. Somewhere between SP11 and 15 the behavior of Synthetic Resources changed to remove the URL suffix and replace it with the component resource type. This behavior is in UrlBuilder in the Sling API. So if you're using a suffix to pass information to your component, it will no longer work. We worked around the issue by using a URL selector instead of the suffix. The SSI feature then creates an include with the selector intact and the component can get it from the URL using the RequestPathInfo.getSelectorString(). Hope this helps!