Cant remove cache from pods in external kubernetes context

I don't know is it a bug or more a question. Cant find an answer to this anywhere.

So I created external kubernetes context
docker buildx create --name portal-cluster --driver kubernetes --driver-opt replicas=4,namespace=docker-buildx,loadbalance=sticky
docker buildx use portal-cluster
Build test image
docker buildx build -t nexus.domain.com:445/portal-notary:434 -o type=registry --push --cache-to=type=inline --progress=plain .
Within builder pod I can see that cache created

/var/lib/buildkit # du -sh *
0	buildkitd.lock
52.0K	cache.db
821.9M	runc-overlayfs

If I run buildx build again, as expected it will use some layers from cache.
But I need in some cases prune cache if disk usage become to large.
I tried execute

docker buildx prune --all
docker buildx prune --verbose --builder portal-cluster
docker builder prune --all

But used space inside pod still the same and at build time layers still cached. Only by terminate pod I can delete cache inside pod.
docker buildx du return

Reclaimable:	0B
Total:		0B

Reproduce the external Kubernetes builder setup with the shown docker buildx create and build commands, then compare docker buildx prune, docker builder prune, and docker buildx du with the cache under /var/lib/buildkit in the builder pod. Determine why pruning reports 0B while cached layers remain, and verify that the documented or implemented cleanup path actually removes reclaimable cache without terminating the pod.