I don't understand this... Have you never had a combat where for whatever reason (noise, waiting in ambush, betrayal, etc.) new combatants enter the fray?
Not that I did not set up before hand. I decide how many monsters are nearby and if they come to join the fray then they do, but no more than was decided ahead of time.
Have you never had opponents who have faced your PC's on more than one occasion... or have henchmen who faced them and were able to relay their tactics and general capabilities to the BBEG so that he/she was prepared for them? The fact that surprise is a part of combat means the space and rules can change...
You mean that in a new combat there are different rules? Sure, that happens all the time. But now you are talking about one combat leading to a second combat, which is far different than talking about a single combat.
Okay putting aside the fact that lost is defined in the D&D DMG as inadvertently travelling in the wrong direction and spending 1d6 hours before being able to check to see if you travel in the correct direction... and doesn't mean... "always knows the route". In fact being lost or not only deals with whether one spends time going in the correct direction not anything around a specific route... Let's put the actual rules aside and...
What if there is? What if there is an impassable river or magical interference or a number of other things that could plausibly be in the wilderness of a world where magic and monsters exist?? The ranger's ability in no way guarantees totally safe passage. the fact you've chosen to homebrew it into that is a self-made problem.
Did that magical interference exist before ranger declared their favorite terrain? Or did it appear after the DM realized the ranger was breezing through the challenge and then added it for the sole purpose of canceling the ranger's ability? Therein lies the difference.
We presented a set-up, and instead of acknowledging the set-up, it was then immediately declared not a problem, because you can change the set-up.
Also, on the ranger and being lost, if I know my destination is to the west, about a week's travel, and I set out, and I am incapable of accidentally going in the wrong direction and losing 1d6 hours... then I will always reach my desitination. I always know the route to where I am going, because I never go the wrong way. I won't say it is the best route, but without magical interference or impassable barriers, you can't stop a ranger from getting from Point A to point B in their favored terrain.
I would consider tailoring your challenges to your players good advice... and I believe, though I could be worng that 5e espouses that philosophy.
That said the "challenge" shifting is an assumption you are making... it easily could have also had a complication added to it or it could have been set up beforehand.
It can be good advice, but too much tailoring just cancels player abilities.
And it could have been something set up beforehand... but in this case it wasn't. In this case we set up the example, and then other people came in and changed it to prove our example invalid. We have the entire conversation.
You're addressing the specific example but instead try addressing the principle of putting the players in a situation where they can choose between that safe hut or something else... an NPC, treasure, knowledge, etc. The specific doesn't matter... What do they value? Put it at risk or up for grabs and suddenly that hut isn't ALWAYS a safe haven no one will venture out of.
And once the thing isn't at risk? Or do you interrupt every single long rest they try to take until they stop casting the Hut?
Every challenge you create for the PC's is a direct action against some ability they have. Seriously you are challenging their capabilities, that is kind of the point of the game. My example doesn't neutralize anything... it offers a choice, that's the challenge.
I never said it neutralized it, because as I pointed out, after they go get the kid, they just head back, recast the Hut and rest again. And unless you have the kid wander off a second time, the Hut still provides a safe haven to rest in.
And, you seem big on choice, but you have to realize that what you are doing by having a kid wander out of the hut, or a monster threaten something they left outside the hut, is you are negating their choice. They chose to sleep in the hut, and now you have put forth something where they can either stick by their decision, and lose something, or follow your plan and not use the Hut.
And if you do this every single time... eventually it stops being a challenge to the player's capabilites and is just you punishing them for making a choice you don't like.
Again let go of the specific example... it's not the point.
Then you should have been clear it was only a general example, and explained the larger point, because everyone has been reacting to the specific example.
But every single one of your general examples runs into the same two problems. 1) They don't stop you from resting in the Hut, they only delay the rest until after the threat is over and 2) they are being presented entirely to force them to not rest in the Hut. What if a dragon attacked their town every time they cast the spell. That doesn't even make sense. What knowledge is being threatened that they know is being threatened after settling down for a long rest? It seems much more like you are just advocating for interrupting the Long Rest, which does nothing to them using the Hut later when you stop interrupting their rest.