Fictize the act of plot generation. What does this mean? It means create a fictional reason for the players to tell you the kind of information you're looking for. It's best to make it something subjective, that way you can either roll with it or change it, and still be "right". Here are some examples:
"Abner, you have a dream that wakes you up in the middle of the night, all trembling and sweaty. It involved your future, and you wake up feeling a need to prove yourself. What was the dream about?"
"Babette, as you're watching the Queen's parade pass by, you flash back to a time you were young. You remember a promise you made to yourself. What was that promise?"
"Caliban, one of the people at the table reminds you of someone in your past who made a great impression on you. They taught you about the importance of following your dreams. Who was that person, and what did they say that you took to heart?"
"Dagon, while rearranging your gear that night, you find a note in the bottom of your pack. You're not sure how long it's been there. It says simply "help me." You think you recognize the handwriting. Whose writing is it? Why might they need your help?"
"Elsie, as you're waiting for the food to arrive you notice a person who enters the room and looks around slowly, as if they're searching for someone. You haven't seen them in a long time, but you recognize the face. Who is it, and what do you think they're looking for?"
"Falafel, the fortune teller reads your palm and tells you that you are in danger of slipping from your true path. There is something you have forgotten to do, some responsibility or detail from the past. What was it?"
You can also throw a question to the entire group for brainstorming. Don't be afraid to ask for a scene about it. Say you just came up with a weird random encounter or event, and even you don't know what it means. That's ok, just sit the characters down and let them have a conversation about it. "Ok guys, so that night as you sit around the campfire, you all begin wondering about (that weird thing that happened earlier). Like... What could it mean? Is there something that should be done about it? I want to hear that conversation."
The players will probably come up with several theories on their own, and you can choose whichever one sounds good to you. Maybe they're exactly right. Or maybe they're only half right, but it will give you something to go on.
If you are looking for an "in-game" resolution Ravenloft, and by extension Barovia, has always had a history of twisting things to its liking. The Demiplane of Dread is quasi-aware after all (or at least was in past editions). There is ample room for the environment to adapt to this advantage and thwart it, the walls can become more and more opaque over time to this ability as it adapts.
Another idea is that it could also change to start picking up "false positives". The ability could indicate the presence of things that are not actually there or at least not perceptible to the others but harmless. Like the spirit of a dead girl could set it off as she watches from the sidelines of a battle. He could freak out at "seeing" something more on the battlefield that can cause confusion and mishaps during combat.
I think that Zeiss Ikon has it right that you can just renege on the gift but I agree that an in game solution is usually best for flavor. Then your issue will be when/if they leave Barovia you will have to figure on a "fix" at that time. Or you can just say that the alterations by the Demiplane were fundamental and can't be reversed. Maybe the sensory overload was amplified by the Demiplane so that his brain can no longer process the input through solid objects?
These are not RAW solutions and simply based on flavor and experience over years of dealing with Ravenloft in books and modules. I personally would have limited that to 30ft, but that is my table not yours.
Best Answer
Your "Curse" is not mechanically a curse
You can impose conditions that would be a curse in a fairy tale, but are not mechanically curses.
Remove Curse is a 3rd level spell. It is meant to remove conditions such as those imposed by the Bestow Curse or Geas or to remove the atunement from a cursed item so that the item can be disposed of.
Remove curse will not remove a the effects of a True Polymorph or a Flesh to Stone for instance. Your custom "curse" with its escape clause is obviously into homebrew territory, but not only are you justified in declaring that it is beyond Remove Curse, the players should not be surprised. Your custom spell is more akin to True Polymorph than it is to Bestow Curse. I have used custom conditions that must be resolved by a MacGuffin frequently, and my players are never surprised when it takes far more than a low level remove curse or dispel magic to break it.
To avoid misleading players, especially those that invest heavily into RAW, you may want to hint strongly that remove curse won't work ahead of time. Ideally, you would avoid the word curse, but since this fits perfectly within "fairy tale curses" that might be hard. Instead, I might have the NPCs say things like it is a "special kind of curse" or "like a curse". You could even have a local cleric say they have already tried remove curse, or at least emphasize the "only" way to break the curse.
Have backup plans and be adaptable
In your particular scenario, you are fully justified in declaring that the mechanical ability just won't do it and any reasonable player should understand that. Sometimes that won't be the case. Sometimes the players will really find some way to short-circuit your plan that its hard to say no to without cheating your players.
That is where you remember that you know and control things they don't. Among other things, remember that MacGuffins are largely fungible. So, they just found a way to avoid something you put a lot of work into planning. You might need to back off for a little bit and let them enjoy that victory, but then some other MacGuffin winds up being in the place that you put so much work into planning and they still have to go, just for a slightly different reason.
In your particular case, if the curse is broken perhaps the princess herself gets carried off to the well guarded location you planned out.