The latest Custom Maps version (1.8.1) should have copied all maps from the CustomMaps folder into the phone's internal memory when it started up the first time. After that point, Custom Maps uses only the maps in the internal memory. This change was required by Play Store which no longer allows apps to read/write the public area of the SD card unless the app is a file management app. Custom Maps obviously is not, so I had to move the maps to the internal memory area reserved for the app.
You could "Force stop" the Custom Maps app and relaunch it, and the maps might appear in the list. If they don't, rebooting the phone might help.
If neither of those actions help, you can open the kmz files in CustomMaps folder one by one through a file manager app like Google Files. In Google Files you open the files simply by tapping on them. If you have other apps installed that can display kmz files, the app may ask you which app you want to use.
Sorry about the trouble. This never happened when I was testing the new version.