iPhone 12 Pro : 4K video exported from iMovie only reflected as 1080p video on youtube

I made a video using iMovie on my iPhone 12 Pro and saved a copy in original resolution f 4K at 60fps. The original source videos for this project were all 4K at 60fps videos and also the final video that was rendered from iMovie on my iPhone 12 Pro. I confirmed the video details (resolution, fps etc.) using other apps (eg Google Photos).

I uploaded the final video to YouTube using the YouTube app on my iPhone 12 Pro. I understand that YouTube takes some time in processing the uploaded videos to its full resolution, but the maximum Quality option that YouTube displayed for this video was only 1080p60 (i.e. Full HD at 60 fps). I checked back after a couple days, assuming maybe the processing to 4K takes more time than I thought, but to no avail. How can I upload the 4K video (rendered from iMovie on iPhone 12 Pro) to YouTube at its full original resolution?