privatemounts = false is required if snap is separate from the backup service.

nixos/modules/nixos/services/zfs-nightly-snap/default.nix

@@ -3,9 +3,7 @@
   lib,
   pkgs,
   ...
-}:
-
-let
+}: let
   cfg = config.mySystem.services.zfs-nightly-snap;
 
   # Replaces/Creates and mounts a ZFS snapshot
@@ -62,8 +60,7 @@ let
       mount | grep "$BACKUP_DIRECTORY"
     '';
   };
-in
-{
+in {
   options.mySystem.services.zfs-nightly-snap = {
     enable = lib.mkEnableOption "ZFS nightly snapshot service";
 
@@ -115,6 +112,7 @@ in
         serviceConfig = {
           Type = "oneshot";
           ExecStart = "${resticSnapAndMount}/bin/zfs-nightly-snap";
+          PrivateMounts = "no"; # We want to mount the snapshot to the system
         };
         requires = ["zfs.target"];
         after = ["zfs.target"];