According mkfs.btrfs manpage the behaviour is expected: -f, --forceForce overwrite when an existing filesystem is detected on the device. By default, mkfs.btrfs will not write to the device if it suspects that there is a filesystem or partition table on the device already.
I guess the change should be made on partman-btrfs/commit.d/format_btrfs -- Melhores cumprimentos/Best regards, Miguel Figueiredo