Were snaps in manifest even a thing for desktop images? I think snaps were never listed there. Take cosmic for instance (https://launchpad.net/~ubuntu-cdimage/+livefs/ubuntu/cosmic/ubuntu/+build/147957) and the manifest file doesn’t list it: https://launchpadlibrarian.net/393750371/livecd.ubuntu.manifest.
This is something we added for multi-layered images in disco (which ubuntu-desktop isn’t due to the pending ubuntu-cdimage and debian-cd MP), but not for “flat” images.
CPC and ubuntu-server-live have their own hook that were written I guess one or two cycles ago to add them (and wasn’t generalized apparently at the time, unfortunately).
Adding to flat image would be trivial though.