Thanks for raising this. There turned out to be a setting that was set for the mobile app that was causing people to need to log in again every 5 days, I've updated it to every 400 days. There are some error states mostly related to poor network conditions that will prevent people from being logged out in the next version of the app.
As a reminder, these apps are volunteer maintained so there isn't a ton of capacity around to look into every problem as they're raised.
@Pete - in terms of why you need to log in, we've moved from storing saved reaches on the device to storing it as part of your user profile. In the future we expect to be able to provide paddlers with the ability to see the same list of saved reaches when logged in both the app and the web page.
For context, We're doing this due to clear user feedback that people want access to saved reaches on web and that they do not want to manage two lists separately. Furthermore, having the source of truth of this data be on our servers rather than on people's phones dramatically reduces the maintenance burden for our volunteer programming team.