I got an answer from ViewRanger support:
The app switches between maps automatically as you zoom in and out.
To stop that, first zoom to the map you want. Then use menu tab, 3 dots top right, Display preferences. Scroll down, switch on "Don' t change scale on zooming". The app then stays on the same map scale as you zoom in and out, until you switch that off or explicitly open a map from the Maps list.