Why aren't my #shorts videos playing as shorts on YouTube phone app?

I've started experimenting with rereleasing short clips of old videos using the new YouTube shorts feature. I've followed all the documentation I can find on the feature, which amounts to this:

I've cropped the video to 9:16 (vertical format).

I've made the video less than 1 minute long.

I've included #Shorts in the title and description.

And yet when the video plays in the YouTube phone app, it still plays like a regular wide video shoved up in the top of the screen, rather than taking advantage of the full phone screen. Videos from other channels are behaving properly.

What am I missing?