FCP reduces exported mp4 resolution without warning

My video project is HD-vertical (1080x1920). After exporting it as an mp4 by using the settings Computer and H.264 Better Quality at the export prompt, I see that the resolution of the exported video has downgraded without warning to 608x1080 - although the Resolution in the export prompt itself was listed as 1080x1920! I'm having to use a third-party mov-to-mp4 converter (like the online Adobe Premier converter) as a workaround to maintain 1080x1920. How do I get FCP to export mp4s in the set resolution?

Square or rectangular pixels? Check the pixel aspect ratio. Going from rect to square on rotate could do that. (Guessing here as I haven't experienced this problem myself.) In Quicktime Player 7 you can rotate, save, and it doesn't re-encode or mandate exporting the video once again.
 

It’s a bug from the preset. When you export re-select the file size and then it should work.

Yep I found the same 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 1080x1920, I changed the dropdown value to 720x1280, then back to 1080x1920, 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.