Resolution of QuickProof-exported files

I recall a claim that QuickProof is supposed to be a fast approach to exporting simple previews due to using internal proxy files instead of ARW directly, so I'm testing that - not sure if the gain is there, but something else caught me by surprise: I have set preview size to 3840px and while horizontal images export at 3840x2560, vertical end up being only 1707x2560, which is way below the target set in preferences. Is this a bug or an incorrect value selection on my side?