What's the word for the facial expression over an unexpected disappointment?

Possibly "doleful" would be appropriate,

doleful adjective: expressing sorrow; mournful. "a doleful look"

synonyms: mournful, woeful, sorrowful, sad, unhappy, depressed, gloomy, morose, melancholy, miserable, forlorn, wretched, woebegone, despondent, dejected, disconsolate, downcast, crestfallen, downhearted. See, Google doleful

Or perhaps, "crestfallen",

crestfallen adjective: sad and disappointed. "he came back empty-handed and crestfallen"

synonyms: downhearted, downcast, despondent, disappointed. See, Google crestfallen

Richard Sherman following Butler's SB interception

Chagrined:

verb

(be chagrined)

Feel distressed or humiliated:

ODO

From 100 Words for Facial Expressions, by Mark Nichol, on dailywritingtips.com:

14. Chagrined: humiliated or disappointed

From A Bit of Blue Ribbon, by Sarah Beaumont Kennedy, in Outing, Volume 27, 1896, page 5:

The girl with the parasol nodded, the marvelous blonde with the chagrined face merely turned her head.

From The Netherfield Affair, by Penelope Swan, 2015:

Elizabeth hid a smile as she saw Miss Bingley's chagrined face.

The text emoticon for chagrined is half grin; half grimace:

:-/

The emoticon immage:

There is no word for such a facial expression, except, perhaps, a grimace.

The feeling is dismay or consternation.

I think "fall" would work but adding "in disappointment" would make it clearer:

His face fell in disappointment.

The link also has some other options for both disappointment (FC122) and embarrassment (FC121, right above).