P3D v4 add-on.xml autogen not working for default.xml

For some reason, I have always believed that the new add-on.xml support for autogen -- merging rather than overwriting as it did prior to 4.4 -- worked with default.xml as well. But now I find that it doesn't, with 4.5 at least, not sure about 4.4 at all now!
I've used this to add my custom vegetation autogen descriptions, and a couple of library objects, but a few users were reporting missing autogen, which I've finally been able to confirm.
This is just a heads-up, plus a question -- did this used to work in 4.4, or has it never worked like the other autogen description files?
This is my experience.

I created an autogendescriptions.spb, an autogendescriptions_Legacy.spb and a default.xml files. All of them have the default entries and some new entries (my custom library objects).
I added them in a folder named autogen. I added this folder as a subfolder in my add-on scenery folder.
This link shows how to do that correctly.
I tested it in default and ORBX environment and it worked fine with P3D V4.5
After that I re-created the files using only mine data (no default data inside)
Same result.
The files were not readable though if I renamed them (i.e from autogendescriptions.spb to autogendescriptions_MYAIRPORT.spb).

The only thing that comes to my mind and may be a problem is the following.
According to the link provided above the users must deactivate autogen configuration merger if it is installed before adding the autogen folder (which I guess means before installing the scenery).
I don't know what happens if a scenery built that way is installed and the autogen configuration merger runs. Of course this action (deactivating autogen configuration merger) may conflict with sceneries built before 4.4 version where autogen configuration merger is needed... :scratchch

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Manwlo, thanks for that. Your understanding of how the autogendescriptions work is correct, I'm just concerned with the default.xml file, which adds the Autogen library objects. The rest works as expected, and there are no problems there. This came to light after a user found that his street lights were missing outside my scenery area, and these were obviously autogen library objects. My own testing was with Orbx NZSI, and by including my own default.xml file (with just a few objects) I don't seem to see any Orbx library objects, such as the 'windbreak' models used a lot in NZ.
By the way, that link is actually my site, and although this seems to have become the 'de facto' source of information on the new P3Dv4.5 autogen merge abilities, I am concerned that I am putting people wrong regarding the default.xml file.
I don't know whether LM didn't intend this to work the same way, or if it is just a bug in 4.5, but it seemed to make sense to me that this file would work the same as the other descriptions -- after all, their SDK does show how to create and add your own autogen library objects, so a simpler way to distribute these would be a no-brainer.
However, if you do have a default.xml file included in your setup, and it does appear to work, then I'd be keen to know that it works for others.
After doing a lot of research for this issue I would like to post the results for further discussion or actions. This will be a detailed post so it may be a little bit long and is related to P3DV4.4 or more capability to automerge autogen without the usage of ACM from Arno.
In order to create custom autogen I created some custom trees, buildings and vehicles. I added them all in a .bgl library file.
Those items were also placed using .xml code in some areas of the airport.
I used the same library files (same GUIDS) to add some of those trees and some of those buildings into a custom default.xml file, a custom autogendescriptions.spb and a custom autogendescriptions_Legacy.spb file.
These custom files were placed in an autogen folder into the scenery (not the default autogen path or not by editing the default P3D files) being read through an add-on.xml file.

Everything were supposed to be working fine. I checked the scenery in a total of eight different PC's with no issues. After the release of the product there were three or four known cases that reported missing autogen. Some of them did not see the autogen from my scenery while some others were missing the autogen from other sceneries. One of the users did not report a problem during beta testing (it was one of the eight machines) but after deleting the beta version and installing the released one (which was exactly the same product).

After a lot of troubleshooting and experimenting with that beta tester it came up that the autogen would show up if some airports were disabled. Sometimes, after disabling an airport running ACM and enabling the airport again the autogen of that airport showed up but it was missing from another airport or area.

I did further investigation and the conclusion was that all the airports or add-ons that were conflicting with each other had one common element. A separate default.xml file.
The next step for me was to deactivate mine default.xml file. Doing that made my WHOLE autogen disappear including the objects that were placed using the autogendescription entries (~70%).
On the other hand as soon as I did so, some autogen trees from ORBX appeared in the area of my airport (outside the photoreal I used). It was the first time I realized that I had the same problem since the beginning but my autogen was "winning" the other autogens so I did not spot the problem.

I also tried to merge my default.xml with the P3D file. Nothing changed regarding the problem. The users still could not see mine or other airport's autogen.

I deleted all the autogen that was placed with the default.xml through the annotator.exe AND the default.xml file from my scenery.
After that all the remaining autogen was present along with the autogen of all other addons .
I then added some more entries to autogendescriptions.spb and autogendescriptions_legacy.spb and replaced all the deleted autogen with new entries. I placed the same amount to check if this was an issue. It was NOT! No matter if I placed more files than in the beginning (~40000 instead of ~39000) everything was showing up.

I 'm not in a position to understand exactly what causes this conflict. I don't know if there was going to be an issue if I used only the P3D default.xml (placing default and not custom items)

However, I 'm in the position to recommend NOT to use a custom default.xml file with custom entries to create your autogen. It DOES conflict with other sceneries. This conflict may not be visible to many users or even to yourself. But the problem is there.

Regards and happy developing! :)