There is no setting to force the app to use offline data exclusively. However, the app automatically prefers the offline map data when it is available.
So, if you have downloaded the offline map for a particular area, the basic map background is taken from that offline data. The app does not need to download the map tiles again, so this significantly reduces data usage.
However, offline maps contain only the basic map and some basic tourist information. Other data is still loaded online, even when you have the offline map downloaded. For example, information about restaurants and other points of interest, photos, opening hours, traffic information, aerial maps, etc.
When you have no internet connection at all, the downloaded offline map is still available, which is the main purpose of offline maps. However, online information is then unavailable, and route planning has to be calculated by the app using the offline map data, without current road closures, traffic information, etc.
So it may look as if the app is still using a lot of mobile data even with offline maps downloaded. But the downloaded offline map does reduce the amount of data transferred — the app simply still needs an internet connection for many other types of information.