restored vm on thin provisioned drbd9 storage is no more "thin"

[mmenaz]

Hi, I've prox 4.1 (enterprise repo) and drbd9 cluster with thin provisioning enabled. I've seen that if I create a VM in drbd9 storage is correctly thin provisioned, but if I restore the VM then 100% (almost) of the space is alloccated.
i.e. I've backedup VM 108 and restored as VM 999, and lvs shows this:

root@prox01:~# lvs
  LV  VG  Attr  LSize  Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  .drbdctrl_0  drbdpool -wi-ao----  4.00m   
  .drbdctrl_1  drbdpool -wi-ao----  4.00m   
  drbdthinpool  drbdpool twi-aotz--  1.42t  27.72  14.41   
[cut]
  vm-108-disk-1_00 drbdpool Vwi-aotz--  8.00g drbdthinpool  8.32   
  vm-999-disk-1_00 drbdpool Vwi-aotz--  8.00g drbdthinpool  99.98   
  data  pve  -wi-ao---- 100.44g   
  root  pve  -wi-ao----  46.50g   
  swap  pve  -wi-ao----  23.25g

Is it a bug or something I'm missing?
Thanks in advance

root@prox01:~# pveversion -v
proxmox-ve: 4.1-37 (running kernel: 4.2.8-1-pve)
pve-manager: 4.1-13 (running version: 4.1-13/cfb599fb)
pve-kernel-4.2.6-1-pve: 4.2.6-36
pve-kernel-4.2.8-1-pve: 4.2.8-37
lvm2: 2.02.116-pve2
corosync-pve: 2.3.5-2
libqb0: 1.0-1
pve-cluster: 4.0-32
qemu-server: 4.0-55
pve-firmware: 1.1-7
libpve-common-perl: 4.0-48
libpve-access-control: 4.0-11
libpve-storage-perl: 4.0-40
pve-libspice-server1: 0.12.5-2
vncterm: 1.2-1
pve-qemu-kvm: 2.5-5
pve-container: 1.0-44
pve-firewall: 2.0-17
pve-ha-manager: 1.0-21
ksm-control-daemon: 1.2-1
glusterfs-client: 3.5.2-2+deb8u1
lxc-pve: 1.1.5-7
lxcfs: 0.13-pve3
cgmanager: 0.39-pve1
criu: 1.6.0-1
zfsutils: 0.6.5-pve7~jessie
drbdmanage: 0.91-1