Potential Fix....
I've had the same issues with my Lezyne Super GPS. I build my route with the Lezyne Root route builder, or RidewithGPS or Strava and then can't get the turn-by-turn directions to work on the Super GPS when I'm riding.
So....after weeks of frustration in trying to figure it out, I went and deleted all of my historical ride records off of the Super GPS. I did this from the Super GPS menu, not the iPhone app. After I cleared off all of my past rides, the turn-by-turn directions are now working correctly during rides.
So now when I come home from a ride, I upload the ride to Lezyne/Strava and then delete it off of the Super GPS so the Super GPS doesn't have any old ride data on it and is fresh for my next ride. This has worked consistently now for a 7+ rides....so perhaps this is the solution.
Dave