Not sure if this is the same issue you’ve found, but once I’ve departed, sometimes the only thing I’ll see are contact center, or request flight following. If I request flight following, and go through the get/acknowledge the code process, then the option comes up to request an IFR plan. Not ideal (or right) but at least it gets me an IFR plan.
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I posted this to Heavy Division discord and they confirmed the issue. However I don’t know if there is a known solution. @KlappDrache728 maybe you could post an official bug in the Heavy Division GitHub repo.
The suggestion I got in the discord was to use VATSIM, or to import the simbrief plan into the MSFS flight planner before starting the flight and then import the same plan into the 787 FMC as well during startup. That does seem to work although if you make changes in the FMC not sure if ATC picks those up. Ideally the ATC would just go off the planes FMC plan. It works that way for A32NX so I can imagine its possible.