Prettier 2.3.2
Playground link

--parser less

Input:

@variable: {
  field: something;
};

Output:

@variable: {
  field: something;
}; ;

(If you test it out in the playground, another space + semicolon are always added, no matter how many are in the "input")

Expected behavior:
I would expect prettier to be satisfied with zero or one semicolon, not always suggest another. See detached rulesets docs.