Thats probably because your HDDs ins't mounted in the mountpoint where you point your directory storage to. So everything written to that directory storage will end up on your root filesystem and that is the LV that also stores your "local" with just 72GB.
Make sure you run pvesm set Your8TBStorageID --is_mountpoint yes or otherwise the directory storage won't handle the folder it's pointed at as a mountpoint and won't fail if it's not mounted, resulting in problems like you see now.