This is a bit of a mystery. Can you try requesting another build from build.snapcraft.io and see if it still happens?
I don’t know build.snapcraft.io well, but I’ve had a look at what Launchpad did on the backend. That snapcraft.yaml is correct, and I can see that LP fetched it when determining whether to use snapcraft from apt or a snap, but for some reason LP decided no base was specified (channels={"snapcraft": "apt"}). I’ve created my own version of the snap recipe on Launchpad which is configured equivalently, and it built fine on amd64.