While there are a number of potential causes for this issue, I imagine it is being caused by you not having the right code to define/set the map area...
If so, the problem could be fixed by making your map script lua file have this code:
and making your map save lua file have code like what I circled in blue here:
Note that the map save file should keep the rest of its code and that the two 1024's should be changed to match your map size if your map is not 20 km x 20 km. A 10 km x 10 km map would use two 512's instead of 1024's, while a 5 km x 5 km map would use two 256's, etc.
The relevant lua files are located in the map's folder and can be easily edited with a program such as Notepad++.