Wow that is strange!

As you're possibly aware, there some size discrepancy occurs when the units MB/GB/TB are used when actually MiB/GiB/TiB are meant is (i.e. bytes to a power of 1000 vs bytes to a power of 1024 respectively). And sometimes is just not clear which unit is being used (e.g. when it just says M/G/T). But that certainly doesn't explain anything even close to 100G (whichever unit it is)!

I wonder if it's some weird interaction between zfs, lxc and/or samba?!

If you can provide some info, I'll see if I can recreate the issue on my local Proxmox server. If I can, then hopefully I can work out what is going on and work out a fix; or at least a workaround. First a few questions:

Where are you seeing this 100GB space being used?
Is from a host PC where the SMB share is mounted If so, what OS(s)?
and/or in pre-installed webUI (WebDAV-CGI) on the fileserver?
and/or somewhere else?

It would also be good to get some info from both the guest and the host.

On the the guest, run these; where:
'/path/to/local/smb/share/dir' is the actual path to the dir that is being shared via SMB
SMB_MOUNT is the name of the share; and ROOT_SMB_PASSWORD is your root samba password (obviously don't post the first code block that includes your password)

smbmount=SMB_MOUNT
password='ROOT_SMB_PASSWORD'
# then install smbclient
apt update
apt install -y smbclient

Then give me the output of these commands:

turnkey-version
df -h
du -sh /path/to/local/smb/share/dir
grep '^/' /dev/null)

On the Proxmox host share the ouptu of these (where VMID is the container ID):

vmid=VMID
pveversion
grep '^/' 

Please also share anything else you think might be of interest, especially anything that might help me reproduce the issue.

Those commands shouldn't include anything sensitive, but redact or exclude anything you are uncomfortable sharing. Although unless it's obvious, please note what has been redacted/omitted.