Yep, Tim, correct - the problem is that we (and I also assume ASOBO/MS) don´t know the rules of loading orders the packages. We only know that it´s not the same as in MSFS2020 because the priority attribute in the content.xml file is not valid as in MSFS2020. We have gotten an answer to our question about how the rules how you can control the loading order of the packages in the ASOBO/MS dev forum and we have added it with 2502, but it seems, that doesn´t work as expected.
So, long story short, we have no clue, at the moment, how we can handle third-party scenes. Personally, I assume that the third-party add-on devs must also follow the rules according to the SDK and the PackageOrderHint selections - but again, I don´t know 
Cheers
Richard