How to avoid alienation by expected but slow mood shift, and still keep players out of spoilers?

I run an MLP campaign. By design, at the start of the campaign what the citizens (PCs) know about the world is true, but it’s not the whole and complete truth, and many issues of the past are either not widely known or just reframed to appear less severe than they are. The campaign revolves around them figuring out How Things Really Are, and becoming ones who keep the surface level of the Utopia running.

And here’s the question. MLP makes people think that they know how things really are. So, over time a player may decide that it’s too dark, or by other means too conflicting with their own vision.

Be it other campaign, we could compare our visions for compatibility beforehand, to make sure that it works.

But this campaign is meant to include perspective shifts; I have a few players that are prone to ‘bleeding’ (and know that!) and/or prefer to stay out of spoilers. The ‘actual state of the world’ has/will have a ‘darker past’; this Utopia is based on a few questionable decisions, and is not as stable as it appears at first. I am afraid of alienating these players, or being met by a reaction of "You asked us to play in the Utopia, and then the mood became totally different". Basically, "I was creating my character for another sort of game, one that you initially described to me; and now it’s a different game, one that I don’t actually like".

How to reduce this risk of alienation, yet still keep the mood of mystery and (classical urban-fantasy) ‘this is deeper than you have thought’, without spoilers?