No Hero Corps Analyst
For me, after the Fog of War bug was fixed I have to open and adjust a map for every toon and reset all my map markers again. What you are experiencing may be along the same lines. In which case, in the lower right corner of your map is the marker menu. From there select which markers you'd want indicated on your map. The difficulty slider NPC's would be indicated by "Notoriety Contacts".
Justice Shall Prevail Where The Laws Of Men Fail
22079 - Best MMO of the Year : "Heroes of Paragon City "
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149433 - The Fall of Mender Moebius : Part 1 : The Beginning of the End
It's a bug that started when freedom launched. All of the map icons that are not normally turned on by default don't save with your game settings when you enable them. If you switch characters or make a new character you have to turn them all on again manually since they are not saved with the load/save feature for client settings.
A few other annoying things don't save either like the placement of the inspiration window and the size and shape of the supergroup and global/friends list. But those were messed up far before freedom.
Friends don't let friends buy an ncsoft controlled project.

Oh. Well. This is embarassingly obvious xD
Thank you for the help, guys.
So, some time ago the blue person marker for the difficulty slider NPCs disappeared off my map. It wasn't too bad since I remembered their location in some zones, so I could still adjust my difficulty, but it bugged me nonetheless, but not to the extent that I was willing to reinstall the game to fix it (since repair had done nothing).
Now I've gotten a new computer since, and one of my thoughts was that, hey, at least the map will display correctly again. But it doesn't, they're still not on the map. So it's clearly not an issue with my map files but with my account - is there a setting where I accidentally deactivated them or something?