ext4 corruption with MPIO and path_grouping_policy "multibus"

Hello,

One initiator is accessing 2 TGT instances on 2 hosts where the same 
block device is shared among them (actually an RBD volume). The 
configuration is as follow:

TGT1:
<target iqn.2017-08.storage:rbd0-1>
direct-store /dev/rbd0
write-cache off
</target>

TGT2:
<target iqn.2017-08.storage:rbd0-2>
direct-store /dev/rbd0
write-cache off
</target>

On the initiator, when multipath is configured with 
'path_grouping_policy "multibus"' and formated in ext4, the fsck fails 
systematically. The output of "multipath -ll" in the MP device :

360000000000000000e00000000010001 dm-17 IET,VIRTUAL-DISK
size=93G features='1 retain_attached_hw_handler' hwhandler='0' wp=rw
`-+- policy='service-time 0' prio=1 status=active
   |- 11:0:0:1 sds 65:32 active ready running
   `- 12:0:0:1 sdt 65:48 active ready running

I not some errors when tgt start :

read capacity (16): pass through os error: Inappropriate ioctl for device
READ CAPACITY (16) failed: Sense category: -1, try '-v' option for more 
information

I cannot reproduce this issue with 'path_grouping_policy "failover"', 
nor whit LIO as iSCSI target and 'path_grouping_policy "multibus"'. On 
TGT hosts, the rbd cache is disabled ("rbd cache = false").


Did I miss something ? Any enlightenment in order to debug further will 
be welcomed.

Debian Stretch
kernel 4.9.65
tgtd 1.0.69
multipath-tools 0.6.4
open-iscsi  2.0-874