I did another flight.
The deletion of the flight plan happens when swapping between the GNS 530/430 stack and the PMS GNS750. Most likely also the autopilot bug was because of this.
Without swapping avionics, everything worked fine. So forget my post above.
Textures may have improved, especially the vertical speed indicator looks perfect, but the background panel and other wall textures of the cockpit really need some love.
So I’m looking forward to v2.0.
The plane itself is really nice to fly.