I can’t reproduce it in free flight, and it doesn’t happen often. It may be connected to changing the flight plan in the EFB and filing it with ATC after accepting the mission, but it’s inconclusive.
Once you do the walkaround and get into the cockpit, it may or may not open by itself.
The doors are closed during the walkaround, but since I don’t use external view during pre-flight and takeoff, I only notice it mid-flight.
The reproduction rate is 0-50%, since it doesn’t happen every time.
I also once had this issue, but as you can see the cargo is still inside. Instead I often had my cargo vanishing while the door was still closed, so I think these are two separate bugs.
For me it actually vanishes after a while. I thought that since it does not affect aerodynamics, I might as well complete the mission, thinking that this is just a cosmetic issue. Then after a while I noticed that the cargo (secured with nets) is gone, so I restarted the mission.