Use "visitmap -2048 -2048 2048 2048" in console for a 4k map. ( Multiple of 1024)"visitmap -4096 -4096 4096 4096" would be for an 8k map and so on.
Takes a while, you will see the process running in the console. You can stop it with "visitmap stop".
This will make it visible on the webmap, but not for all players in their map menu.
They did most probably, cause it gives some perfomance gains running the server. If you have 10 people heading out different locations and exploring new tiles of the map it might give some stutters, cause the server has to calculate the chunks.
If you do it before all happens, then it should be more fluid. For a small map maybe not necessary, but if you have larger maps you might want better performance.