1.12 Forge. Whenever I exit the game and come back in, the underground map is never saved. Overworld map is saved fine though.

underground map is never meant to be saved, it functions a bit like a sonar so it only shows you whats around you at that time

it's supposed to be greenish without detail...

as for not saving i don't really have a opinion about that my guess is that the original author didn't see a way to save it properly.

the underground map can have different maps for different levels underground so it's hard to differentiate between that when saving and loading.
best approach would be to slice it in height layers but then rebuilding the map as for what is near you will be hard.

i dont have any intentions about adding a feature to save the map at this time. if you have a good idea how to implement it i might think about it

When I use the Underground map it shows whatever is near you but also shows you everything that you explored underground too. It isn't like a "only what's around you sonar" kind of thing. It works exactly the same as the overworld map. It just doesn't save between sessions.

Is there a reason you wouldn't want to save the underground map between sessions and make it functionality the same as the overworld?

"Not meant to be saved"...

Why not? Every other map saves it, and there's zero reason not to other than to be annoying.

I guess if you don't have the skill to do it then there's other map programs that do. Oh well.

I think the primary challenge is that is not just a single underground map, but its many maps depending on how deep underground you are.