If you will be riding in areas where you are offline, the best practice is to switch off the automatic route recalculation. Any alteration to the route in the planning mode will trigger the recalculation - this is what you are experiencing when the route disappears. Route recalculation currently requres you to be online, as the router is located on a srever, not on the phone.
You can save the routes for offline use by exporting them to “.kurviger” format. Currently it is not possible to automatically resynchronize your cloud storage locally for offline use. I hope this comes.
Also explore the usage of “Route history” function - it is a quick way to have your routes available offline. I personally do not like to rely on this function as it apparently has some “special cases”, where it does not behave as expect it to, but most of the time it is fine.