Prettier v3.8.1
Playground link

--parser babel
--tab-width 4
--no-semi
--experimental-ternaries

Input:

condition ? ifTrue
: [
      // Hello, world!
  ]

Output:

condition ? ifTrue
    // Hello, world!
: (
    [
        // Hello, world!
    ]
)

Expected output:

condition ? ifTrue
: [
      // Hello, world!
  ]

Why?
Prettier should not duplicate the comment. I don't really have any specific expected output (other than preserving the fact that the comment is only written once), although wrapping the array in parentheses seems unnecessary.