The phrase "was disgruntled" is a perfectly valid and usable phrase in written English.
You can use it to describe a person who is feeling unhappy or discontented. For example, "The employee was disgruntled by his wages and refused to work any longer."
Dictionary
was disgruntled
adjective
Unhappy; dissatisfied
synonyms
Exact(35)
Lohse, it was decided, was "disgruntled" and a "criminal".
"He was disgruntled, and that's just not Vince Carter's personality".
As in Guinea, the source of the trouble was disgruntled soldiers, the Niger government official said.
Even before the debacle, the work force at Renault's Technocenter research facility was disgruntled.
My neighbour was disgruntled at the interval but left at the end intrigued and convinced.
"He was disgruntled and upset and he wanted money he felt he was owed".
Similar(25)
Investors are disgruntled.
And many Peruvians are disgruntled.
Rural Angolans, especially, are disgruntled.
They're disgruntled, jealous.
They have much to be disgruntled about.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com