Project resolution is also Vertical 1080×1920.
FCP and macOS are up-to-date (10.6.2 and 12.3.1 respectively).
Edit: I found a suggestion that it might be a bug and to try re-selecting the desired resolution in the export prompt. Since on the prompt dropdown it was already set as 1080×1920, I changed the dropdown value to 720×1280, then back to 1080×1920, before exporting. Immediately I saw that the estimated size was significantly greater than before the switch. And it worked! Exported resolution was vertical-HD. Reported the bug to Apple.