I have the same problem. I cache them, I can see them, they are there for my next flight (always in Airplane mode / using iPad Mini4 and the latest GO app version), and then the next time I go to fly, all maps are gone again. I must have cached and re-cached them a dozen times or more. Using latest version of everything.
I cache the maps by turning off Airplane mode (obviously) , and connecting to the internet via WIFI indoors, and opening the GO app, clicking on Enter Device, pulling up the full-screen map, and then zooming in all the way on the location I'm going to be flying and scrolling all-around, allowing it download all the detail and street names of my entire flying area. It works great - for 1 flight, then I have to do it again next time.
I'd love to hear a solution to this.