The OkMap generates all JNX files with the same fix internal ID value (83640174-1324-9471-8501-739457104736) in your header...
This value is used by Garmin BaseCamp to uniquely identify each map JNX, so it can not be displayed simultaneously in BaseCamp, two or more maps JNX with the same ID value...
Each map JNX generated with OkMap must have in your header a unique, exclusive and different ID value...
Ok, I've solved the issue. Now OkMap will generate an unique GUID for each map. Please wait until I'll publish the next version (ASAP). Thanks for reporting.