So, recently saber snap got arm64 support. We used the shorthand to build it in the store. But, snapcraft builder is building only for amd64.
I’m seeing the same behavior:
The shorthand notation:
architectures: [amd64, arm64]
only triggers an amd64 build on snapcraft.io.
Whereas the explicit notation:
architectures:
- build-on: amd64
build-for: amd64
- build-on: arm64
build-for: arm64
triggers a build for amd64 and arm64.
I have no idea I actually even thought the shorthand way was dropped for core22 snaps, I was not aware it would be supposed to work 
https://snapcraft.io/docs/architectures
It’s specifically mentioned here for both core20 and core22 and was suggested to me by upstream. So, I thought to bring it up here.
Yeah, it was supposed to be dropped but was not, which is my mistake.
There are more details in this bug report: Bug #2042167 “remote-build doesn't support shorthand architectur...” : Bugs : Snapcraft