Thanks for letting me know how you solved your issue with the missing maps.
But I don't recommend staying with version 1.7.1. Due to Google Play Store rules, no future version can include permission to read external files. Version 1.8.1 will even be the only one to be able to migrate the maps automatically. Any later version cannot touch the CustomMaps directory on the SD card in any way (again, Play Store rules, not mine). But this also means that no future version will request permission to access external storage for reading or writing.
I have improved the file opening capabilities of Custom Maps, so that many file managers like Google Files (but unfortunately not Samsung "My Files") can open the map files in Custom Maps just by tapping on them. At that time the map gets copied into the internal storage of Custom Maps, but the original is left untouched. Also maps stored in Google Drive can be opened using the Google Drive app. So you could move your map collection into a Google Drive folder, and then load the maps from there into Custom Maps when you need them. Then you don't need to worry about losing your maps.
There is also an additional benefit of having the maps in the app's internal storage. If you want to move Custom Maps off your internal storage to SD card, now the maps move there with the app. Previously the maps were always stored in the emulated external storage (which in practice is public internal storage).
Also, Custom Maps 1.8.1 now has a menu option "Export all maps" in the map list. That menu item allows you to transfer all currently available maps from the internal storage to Google Drive or some other app for safekeeping. I've tried to make this change less disruptive by adding these options for importing and exporting the maps stored in the app.
Author of Custom Maps