The "Mad Mage", Halaster Blackcloak, is a specific character from the Forgotten Realms setting. "Undermountain" is a special place that he has surprising control over as it has been slowly created by him over the century/centuries, and its layout both isn't permanent and isn't there to serve any given group of people. Monsters and sentients sometimes move in, at their own risk.

The map as shown is meant to represent its existence at whatever point in time the adventuring party happens to arrive at. And Halaster keeps a remote eye on anyone of interest who turns up. GMs are encouraged to modify the presented material in whatever way suits their interests.

It tells a story of a "Mad Mage" very well

That Mad Mage just isn't making sense!

It's like a purposeful troll on a group with a persistent completionist mentality.

When (if) they catch on the DM can help expedite their routing.