Since time immemorial, we have this option in the lb config command: https://github.com/vyos/vyos-build/blob/current/scripts/image-build/build-vyos-image#L621

It doesn't seem that we have a reason to use it anymore. I can't remember why exactly it was added, but at the moment APT certainly doesn't consider it a downgrade if you are simply installing a version of a package older than the newest version available in all enabled repos. If the package wasn't installed already, it works as fine without any special options — it's only a downgrade when an older version is replacing an already installed newer version.

From cursory testing, removing that option has no ill effects on the build, and may prevent odd behaviors in the future.