When working with mount points and not all mount points support snapshotting, then the whole LXC won't allow you to snapshot.could be the additional mount point on the container pointing to other disk folder? no .. i think.. other CT are without mountpoint and..snapshot isn't available.
Please show us your actual storage config:...if i covert vps on zfs to qcow snapshot is on, but if i try to move to btrfs it give me only raw option without snapshot, it's normal?
cat /etc/pve/storage.cfg
dir: local
path /var/lib/vz
content rootdir,snippets,images,vztmpl,backup
shared 0
btrfs: local-btrfs
path /var/lib/pve/local-btrfs
content iso
dir: zfs-proxmox-data
path /zfs-sas/proxmox-data
content vztmpl,rootdir,images,snippets
prune-backups keep-all=1
shared 0
dir: zfs-sata1-proxmox
path /zfs-sata1/proxmox
content rootdir,snippets,images,iso,backup,vztmpl
prune-backups keep-all=1
shared 0
dir: Da-Hetzner
disable
path /zfs-sas/file-storage/AAA_HETZNER_PROXMOX
content backup,vztmpl,images,snippets,rootdir,iso
prune-backups keep-all=1
shared 0