Just to share for those who are interested...
The missing subdirectories are in reality accessible by using "cd subdirectory" (with exact name) on an SMB Client. So, it's as if some directories have not "read access" but only "execute access".
After looking in ssh (from where I can see every folders/subfolders without any issue, so it's not a "9p" issue for me) the permissions and others ACL on all the directories/files (previously reset via OMV plugin... but did it applied everywhere ?! Not sure), I can't find a single difference...
So, I don't know why I have this strange behavior.
By the while, I manage to get it work using a dirty solution => mergerfs !
I put my 9p folder /mnt/data (the one mounted from the underlying Proxmox ZFS) in mergerfs. Yes, he's alone...
Next, I used the created mergerfs to set my shares and voila... It works "normaly' => I can see all my folders.
Any idea/explanation on why it's working with mergerfs an not with shareroot plugin ?!
I just have OMV throwing me some errors on some shares when I apply config (same error as in #11).
It looks like it's only for the shares that are not at the root.
If I share "/dir", I have no error (OMV doesn't try to put any ACL, I think ?!)
If I share "/dir/subdir", I have the error (OMV tries to put a "execute" only ACL on "dir" from group users... Why ?! Also, I don't know why it's not working because it should ; And, finally, look at my 2nd phrase in this post...)
Well, it's not really a wonderful way of doing things but, without help, I don't know what to look at to understand why I have these kind of issues and how to do it a better way.
As every reads I made about passing ZFS to VM, neither Proxmox, neither OMV (or others...) wants to make it easy to work and every one is staying at his position... "blabla Proxmox on a baremetal" & "blabla NAS on a baremetal"...
Adding 9p on remotemount plugin won't be possible ?
I'm going to look a way to activate SMB shares directly from Proxmox (ZFS Shares). Should be more "natural"...