In this sentence, what could be added to say that the result was contrary to the person's intent/expectation?
The meeting was planned to show that the board value employees’ opinions, but (it) _______ .
backfired:
to have the reverse of the desired or expected effect
their plans backfired
(Cambridge dictionary) counterproductive adjective uk /ˌkaʊn.tə.prəˈdʌk.tɪv/ us /ˌkaʊn.t̬ɚ.prəˈdʌk.tɪv/
having an effect that is opposite to the one intended or wanted:
- Improved safety measures in cars can be counterproductive as they encourage people to drive faster.
did a U-turn, perhaps?
Per Wiktionary, the word means
A reversal of policy; an about-face, a backflip.
The government have done a U-turn and are now supporting greater European integration instead of opposing it.
You could modify outcome with an adjective like suprise; this is in fact the example given by Merriam-Webster:
outcome:
something that follows as a result or consequence
a surprising outcome