Is API head filter even working?

Hello, i’m trying to fetch PRs by head attribute.

I’m doing :
curl 'https://api.github.com/repositories/284089292/pulls?state=closed&base=bpf-next&head=series/357901=>bpf-next' | grep head -A 2
I’m expecting:
the result will contain only closed pulls sent from head “series/357901=>bpf-next” to base “bpf-next”
like in UI:

What i’ve got:
i’ve got all closed PR for base “bpf-next”.

inb4 - i’ve tried to urlencode, the https://api.github.com/repositories/284089292/pulls?state=closed&base=bpf-next&head=series%2F357901%3D%3Ebpf-next it makes no difference.