Yeah it's a known issue which the team is aware but won't be able to tackle directly until early January due to the holiday break.
For the time being, I made a mod that will fix the issue. You can download it at https://www.galciv4.com/mods/manager
Look for BugFix Singular Map Gen V1.0 for the fix.
To install and use it, simply unpack the zip file content in your mod folder found at %userprofile%\Documents\My Games\GalCiv4\Mods and enable mod in the launcher by checking "Enable game mod support" in MODS. Also make sure in game that Mods are enabled in the option menu under gameplay.