I got caught with this bug too but found a work around mentioned somewhere.
After you land and stop from a successful leg you get the completed screen with a NEXT button. Press this then select the option to CONTINUE rather than exit back to the menus.
You should then find yourself ready to take off for the next leg with your flight plan still loaded. Then you can exit to the menus and do what you like. When you start off again to continue this bush trip leg you should be exactly where you left off, just ready to take off with the flight plan still loaded.
Although this appears to work OK for me I couldn’t find any way to recover a flight plan once it was lost other than deleting all the progress in the Missions folder and restarting the trip again.
Hope this helps.
G