Indeed, this seems to be caused by the simulator itself. Here are my thoughts on the issue:
I have attached two flightplans (exported from the simulator) that both have this issue. One contains a D0 as well as a DLast waypoint, and the other only contains an extra D0. I have also seen cases where there have been a short series of waypoints following this format, for example D0, D1 etc.
KJFKDEEZ.PLN (19.0 KB)
KSEAKBNA.PLN (12.6 KB)
Investigations
What all of these flightplans have had in common is the fact that they contained vectored approaches, where the simulator cannot predict the path from the airport to the first waypoint in the SID. I’m guessing this applies to STARs as well, but I have yet to see such a scenario. After trying some of the flightplans mentioned here, it seems like it does not like procedures that include VOR intercepts either.
Conclusion?
It is my (somewhat educated) guess that the simulator adds these waypoints in order to handle those kinds of procedures for some reason. You should never get this issue if you don’t let the simulator do any changes to your flightplan.
To prevent any changes from being done, do not change anything on the world map screen after importing a PLN file made somewhere else, such as Simbrief or Navigraph Charts. If you change anything, like the selected approach or the departure, the simulator will re-calculate the whole route with its own way of doing so. This completely erases every trace of the originally imported flightplan.
I tried recreating the routes from the attachments above using Navigraph Charts, and was successful in loading the resulting PLN file in the simulator without any Dx waypoints. In order to do this with Simbrief, you have to load the flightplan (again, without changing anything in the world map) and add the SID/STAR through your aircraft’s FMC/MCDU or similar.
Here’s hoping that this helps someone not pull their hair out!
EDIT:
Also, as @DreadfulHydra50 mentioned, playing around with the system locales seem to work for quite a few people. There seems to be at least two ways of doing this, one is already mentioned in this thread and here’s another one:
https://forums.flightsimulator.com/t/flight-plan-misleading-sid-star/158640/2
Kind Regards,
Malte @ Navigraph