Hello Berta,
The current version of Custom Maps app no longer uses the CustomMaps directory due to new Google Play Store rules. Only file management apps are allowed to access directories on the phone's storage. So I had to stop using that directory for storing maps and instead I had to start keeping the maps in the "hidden" internal storage available only to Custom Maps app.
As you have discovered the maps can be still imported into Custom Maps app by opening them with a file manager app (other than Samsung's "My Files" app). In addition to "File Manager Plus", at least Google's "Files" app also works to open the maps in Custom Maps.
I did implement a feature in the current 1.8.1 version of Custom Maps that if the app has storage permission when it is launched the first time on a device, it tries to look for map files in CustomMaps directory and import all of them seamlessly into the internal "hidden" storage. Unfortunately, Google's Play Services has started turning permissions off for apps that have not been used for a while (which I assume is the case often with Custom Maps app). So in many cases Custom Maps cannot automatically import the previously existing maps, and the user has to discover that they can import the maps by opening them with a file manager (other than "My Files").
I am working on improving features to import and export maps from Custom Maps, so that the maps management would get at least a bit easier. Unfortunately the old way no longer seems to be possible with the new Google Play Store rules.