Weather Update:
I tried unpacking the snap, manually dropping the snap.yaml into kernel/unset/meta/snap.yaml and repacking it (like this), but it had no effect, because I noticed that for some reason, snapd was looking for the file, not in /snap/kernel/x1/ or /snap/kernel/current, but just in the /snap/kernel/ directory, which (and this is an uneducated guess) would never contain that file. Though, things might be different on initial boot.
There’s no way this is a bug, is it?