Component not always applying the priority

This doesn't look like a priority issue but about the fact that PDFExportScriptService directly injects PDFExportJobRequestFactory which means that:

1. The component implementation is only used when it comes from the same scope, so farm scope
2. The component implementation isn't updated when an extension is installed

It is not clear to me if this is a real bug or not as it might not be designed as an extension point, and in particular not as an extension point at wiki level. Most components in XWiki are injected in that way and aren't really designed to be replaced by an extension (they can be replaced, but it's not really intended by the implementation). If we changed this systematically, this could have a significant impact on performance as dynamically resolving components comes at a cost. So it can be argued that this is working as expected.

I think the most important question is what you're trying to do in the first place to understand if this is the best mechanism for this.