I also use Btrfs with Overlay2 for Docker.
The first insight is that copying new files via --reflink=always works (see is_reflink_possible). But not copying files that already existed when the container was started.
Maybe the read-only layer and the top read-write layer are on different filesystems? It looks like it is a problem with my Docker setup and not related to the SDK. If I find a solution I will add it here.