Steps to reproduce

docker buildx create --use
docker buildx bake child # Populate cache
docker buildx bake child # Note cache used for parent/child in this build
docker buildx prune
docker buildx bake parent # Note cache used for parent
docker buildx bake child # Note cache invalidated for child, because of COPY in parent

Now, remove the COPY instruction from Dokerfile.parent and observe that the cache is not invalidated for the child when re-running the above steps.