We are having exactly the same, very frustrating problem in Digimesh. We are using Raspian Buster, Raspberry Pi 3B+ mounted on an ARPI600.
We are doing exactly what the tutorials call for and cannot get a consistent result. One boot will work, the serial port will open and the radios will mesh. Other times, the program will fail and will not work even in a loop.
It seems the technology simply does not work reliably on this configuration, and we do not know why. Setting to a bypass mode is unacceptable.