The phrase "a little disappointing when" is correct and usable in written English.
It can be used to express mild disappointment regarding a specific situation or event.
Example: "It was a little disappointing when the concert was canceled at the last minute."
Alternatives: "somewhat disappointing when" or "a bit disheartening when".
It's actually a little disappointing when Guthrie's real voice, a nasal whine, is heard on the program via old tapes.
"It's always a little disappointing when you can't perform at your best, but I'm very proud to be here in such an immense final," he said.
10.39am GMT James Woods is speaking about his fifth place performance... It's always a little disappointing when you can't perform your best but I am incredibly proud.
But "crowdsourcing" is an increasingly respectable method for data-gathering, so it was a little disappointing when, shortly afterwards, Pontifex made clear that the question was rhetorical, and that his true plan was to become another of those Twitter aphorists: Thanks for your contribution.
"I do find it a little disappointing, when you look at what we've done in the cups, and instantly you're asked questions [about your future].
Nobody at the show disgraced themselves onstage, which is always a little disappointing, when it comes to writing it up.
In Kermit's case, however, the answer is more functional than ideological, which is a little disappointing, like when you learn that the gargoyles are really just drain spouts.
Of his batting Wisden said "Trott's average is a little disappointing, but when a special effort was required he was not often at fault".
It's actually a little disappointing to me: when Microsoft implied that they had designed their OS so that copy and paste would be made redundant by their cleverness, I was skeptical but intrigued.
At the zoo, the lion exhibit is considered a highlight for children and parents alike -- so it is always a little disappointing for them when they see first-hand that lions spend 20 hours a day sleeping.
When Mr. Brown merely pleases, it can seem a little disappointing.
