Learn English – idiomatic expression for a promise that was not delivered

Perhaps you are facing the same problem as a Buridan's ass - an ass caught between two similarly inviting choices.

Morton's fork is similar, but with both choices undesirable.

In American English, we often call this having a "deadpan" expression:

deadpan adj
Impassively matter-of-fact, as in style, behavior, or expression: deadpan delivery of the joke.

TFDO

Etymonline gives this origin:

1928, from dead (adj.) + pan (n.) in slang sense of "face."

So it arrives via a description of an absolutely lifeless expression, giving nothing away.