Bridge Slave as netifd Proto

Hello together,
I'm currently trying to realize a bridge slave as netifd proto.
An addition of the interface into the bridge-config is not feasible for the use case.

Has anyone done that before?
Until now I came up with the following config (extract):

proto_{proto}_setup() {
        local config="$1"
        local ifname="$2"

	proto_init_update "$ifname" 1
	proto_send_update "$config"

	ubus wait_for network.interface.{bridge}
	ubus call network.interface.{bridge} up
	ubus call network.interface.{bridge} add_device "{'name':'(ifname to be filled in here)'}"

proto_{proto}_teardown() {
        local config="$1"
        ubus call network.interface.{bridge} remove_device "{'name':'(ifname to be filled in here)'}"

        proto_init_update "*" 0
        proto_send_update "$config"

The ifup works without any problems, the interface is added as slave to the {bridge}-network.
Unfortunately, the ifdown does not work.
I see netifd: {config} (29458): Command failed: Permission denied when the interface is brought down. It seems "proto_send_update" fails, the interface is removed ("nomaster") from the bridge, but still up.

Does anybody have an idea, what the problem with interface teardown could be?