Well, in that sense you DID debuff the ranger in my opinion... nothing on the book said the region was magic, and even the "natural explorer" feature doesn't stop working if the region is magic, she loses the feature if there is a way to get magically lost. I'm fine with this ruling on the table, but its, to me, a way to get around the rangers power, and so debuffing it. I prefer changing how the power works to decrease the rangers power previously, so it is "fair" to the player, as he is aware the feature will not work as RAW, but if debuffing him mid session works for you and your table, more power to you
As a DM I'll do what I damn like, but that's not the point here. The point is that mechanically, the Ranger feature element - "
can't become lost" - is thwarted by "
magical means". Per RAW, what constitutes such means*? Are there any spells or magic items that make you lost? Are there any places in any official content that are designated "
magically confounding" in just the right way? Or is it always up to the DM to make such rulings?
That said, if the player doesn't have the Underdark (or you could rule, the Darklake) as a chosen favoured terrain, they can get lost just the same. Is the DM debuffing the feature if they don't
always set their campaigns in that terrain type? That would be like arguing that the DM is debuffing Fly unless they always set their games under open sky or lofty ceilings.
The ranger is aware that she is either not in her favoured terrain, or if she is in her favoured terrain then she is either not lost or she is subject to magical means. That is not different fundamentally from the DM deciding that yes, the Orc archer does loose an arrow at the Wizard. All are equally fair. Getting back to the mechanics, it would be debuffing the ranger to strike the feature element, because then the ranger can become lost...
wherever.
*In the case of the Darklake, IMO the DM's choice given the wording on PHB 183 is to make the whole special Kuo-toa navigation thing nonsense or count OOTA 39 "
better to trust a kuo-toa guide than attempt to traverse the Darklake alone and become lost within it forever" etc as a hint toward it being a magical region. On that same page it references "
secret patterns" of the Underdark. Overleaf it continues "
Experienced navigators know how to find and read ancient duergar runes...". That's nothing to do with Ranger really, it's Cartographer that makes all that mystery underwhelming. Not to mention Outlander and what is that other background...?