How does disillusionment not qualify as a form of being upset? Google defines "disillusioned" as, "disappointed in someone or something that one discovers to be less good than one had believed." I would absolutely call disappointment a form of being upset.
However, many players don't have a positive experience when they learn how the sausage is made. That's why you don't tell them when you reskin an earth elemental as the powered-up form of the kobold priest.
I have told my players this and they were delighted at how small tweaks can change the impact of an experience. I did not tell them in play,
because DW explicitly says "Never speak the name of your move" for GMs, but I told them after.
People only get upset if you either (a) make a big deal about "the sausage" ingredients while it's literally in front of them, or (b) maintain a falss pretense of things being perfectly custom when they are not. No false pretense, no negative experience. That's...sort of my whole thesis here. Not revealing every single detail instantly is fine; keeping secrets about how the game works from the IRL players is not.
And why you don't list all the treasure that the players neglected to find.
Wholly different subject. I see no connection between this and secretly invalidating the results of player action(s.)
Or tell the players that the NPC that they were certain was a sinister agent was really an agent from their mentor trying to aid them.
They will find this out naturally through play. No need to taunt them with it. Completely unrelated to discovering fudging.
Or that the "cursed" sword they were carrying around was a sentient +5 holy avenger trying to get the paladin to touch it for 6 seconds.
Not really sure why this wouldn't just...happen? Sounds to me like a pretty boring game if incredibly interesting situations are never allowed to occur because of one tiny misconception with zero chance to ever correct, since, y'know, IRL people do this incredibly dangerous maneuver sometimes referred to as "changing one's mind"...
The fact that a DM should keep secrets should not be mistaken as a reason to condemn different DMing styles.
Again, this conflates two completely
One is faking rolls (or equivalents, e.g. boosting monster HP so it survives long enough to do a cool thing or whatever), which involves deceiving the players
about the game they're playing.
This deception not only must be preserved so as to not upset ("disappoint," whatever) them, but to prevent the game itself from unraveling should the tactic ever be discovered. The players must never be allowed to know it happened, or trust may be irreparably lost.
The other is having facts or information within the world
which the characters
are not necessarily aware of and which will not be simply volunteered to them without reason. This is not even deception; it is simply a fact of playing a game where only one person can metaphorically "see" it (the GM), and everyone else must ask questions of that person to reveal things about it. By the very nature of playing a game structured in this way, there must
be information the characters do not have but would like to know, and which the players must uncover through their actions.
To keep such information not merely secret, but inaccessible (essential to fudging), would be a pretty big problem, because playing the game critically depends on that accessibility! But...and this is utterly vital...the fact that the players have a genuine chance
to get that info does not, in any way, require that they MUST get it. They could simply miss it! If they do, that's a consequence that can feed into future events. Even the missed treasure from an adventure! The fact that it was missed can become a future plot point, and lead to more story. Fail forward: even if PCs fail, the story
advances. It just may not advance in the direction they want it to!
GMs having information about the world
that the characters don't is essential to play. GMs having information about how the game works
that the players don't is not at all necessary for play.