I created a number of vertical videos (2160x3840) in After Effects rendered with the lossless preset. From there I used the YouTube 4K preset in Media Encoder to prep the videos for uploading to YouTube.
With the videos now uploaded and playing back vertically beautifully and in 4K resolution on desktop, to my surprise they only play back on mobile (I've only checked iPhone) in up to 480p.
I also tried rendering at other resolutions to see if it was a 4K issue. I tried full HD and 1440p and got the same low res results on playback from mobile.
Could this be a render settings issue?
p.s. I have tried to seek help from YouTube but they don't seem to think it's a YouTube issue, although they weren't too helpful in general.