as shown

I'm using an Earth map.
However, the "/map fullrender" rendering process only renders most of the map, the rest of the map is rendered when the player runs over it.

Your world is missing some data required for squaremap to render those chunks, earthmaps are pretty notorious for this.

Whenever a player visits an area your server will automatically generate this data, hence why squaremap will only render the areas a player has visited

The easiest fix is to use a pregeneration plugin like chunky to generate this data for you without having to visit those areas ingame

See above explanation

Understood. Thank you for your answer. ：)