The phrase "a letdown when the" is correct and usable in written English.
It can be used to express disappointment regarding a situation or event that did not meet expectations.
Example: "The movie was a letdown when the plot took an unexpected turn that made no sense."
Alternatives: "a disappointment when the" or "a letdown at the moment when the".
I was worried about a letdown when the cameras left.
Always a bit of a letdown when the bloke who comes in fifth wins.
Try getting their attention in a week that is ripe for a letdown, when the Jets, after a week without a game, face a 1-3 that they they are expected to beat soundly.
It's a letdown when the evil in the story turns out to come from the most expected corner (an old Nazi, complete with a cane, a fearsome German nurse and a suitcase full of American bills).
"He had a letdown when he was 19, when the ski team didn't pick him up," Dennis Rahlves said.
So it was a letdown when I arrived and found the menu consisted mostly of the typical upscale Italian/New American offerings: mussels, salmon, tuna, shrimp and crab, veal and steak, pork tenderloin and chops and one chicken dish.
And so it is the tiniest bit of a letdown when we show up at Buckingham Palace for the wackiest fart-jokes since Blazing Saddles.
After playing three solid matches in a row, in past years you could have almost guaranteed a letdown when Tipsarevic faced Juan Carlos Ferrero in the fourth round.
That's why it was sort of a letdown when he came to this conclusion about the power of relics: "The sacred objects we prayed to for miracles, it turns out, were when you really boil it down–just vehicles for our minds to work that power".
So, does he face an emotional letdown when the madness is over?
But it's such a letdown when you move from his complex energy to the brittleness of Wyndham Lewis or the stodge of Graham Sutherland – like visiting Portmeirion straight after a trip to Italy.
