CIFS-backed volumes don't work inside a Sysbox container with Shiftfs

I have detected a suspicious behavior around CIFS shares:
I used portainer UI to create the volume with the following options:

type: cifs
device: //127.0.0.1/dev/legacy
o: username=dev,password=dev,vers=3.1.1,uid=1000,gid=1000,cifsacl,mfsymlinks,cache=none

but the problem is reproduced with default configuration too.

If I mount such share are read-only, it works fine, when used by a user with ID 1000 inside the container run via sysbox-runc.
However, when I switch to the read-write mode, I see the following behavior:

1. Attempt to create a file is successful(for example echo 33 > 42.txt)
2. Attempt to copy a file from outside of the volume results in File exists
3. Attempt to copy a file from within the volume results in File exists
4. As a result of operation above, file is created anyway but has size = 0. When I repeat copy operation, it succeeds(size matches, content too)
5. The issue is not reproduced if I stop all containers that used the volume via sysbox-runc(to get shiftfs off it) and then attach it to a container ran by casual, default runc

This is likely related to shiftfs but I have no idea how to debug the issue - kernel module seems to have no options whatsoever.

I am on:

 cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.1 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.1 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

and I used latest release of sysbox, portainer and docker.

Any ideas how to deal with it?