@MarcM, There are a few requirements for complete offline use of the app:
you need to have maps downloaded for offline use
Downloading maps offline is always a good idea, because that potentially saves a lot of data traffic.
the route you start needs to be offline
That is, if you have no active internet connection, else it will just use that, even with offline mode enabled. If you set "auto-download of routes" to enabled, you don't have to worry about routes being offline, they just are once navigate them.
the app must be in offline mode
If you are not in offline mode, you will be asked if you want to. But then if you have no offline maps, you will be asked to go online.
I happen to know that work is being done to smooth the experience, but with these rules, it just works.