Giving access to a block storage directory to a chroot user

My web users are jailed in their home folder by chroot. I need to give them rw sftp access to a directory on a block storage volume (mounted at /mnt/vol_01, thus outside of their home directory). All permissions are recursively set rw for user:www-data and I also set the same ACL permissions. For testing, I already tried to mount --bind the directory on the BSV inside the home folder of a user. Though the sftp user can read the files on the BSV, the uploads fail, with a sftp protocol error -31, even so, a file is created, but he gets a zero byte size. On the other hand, sftp account logged as root user will work with no issue.

How to fix the issue and give read/write access to the BSV directory to the chroot users? Thanks in advance for any clue :)