Lxc copy --refresh occupied storage differs

Two hosts with LXD 4.9 on Ubuntu 20.04 installed via snap. Both use zfs as storage backend.
Daily backup with
lxc copy container remote:container --instance-only --refresh

The storage used differs greatly, some containers take 3x the space (webserver), some seem to miss stuff (isp)

+----------------+------------+------------+
|      NAME      | DU LOCAL   | DU REMOTE  |
+----------------+------------+------------+
| acmedns        | 905.47MB   | 1.74GB     |
+----------------+------------+------------+
| dbserver       | 793.22MB   | 1.37GB     |
+----------------+------------+------------+
| isp            | 44.91GB    | 28.28GB    |
+----------------+------------+------------+
| wordpress      | 1.32GB     | 2.06GB     |
+----------------+------------+------------+
| cav            | 578.20MB   | 619.76MB   |
+----------------+------------+------------+
| mail           | 84.89GB    | 81.71GB    |
+----------------+------------+------------+
| webserver      | 32.08GB    | 95.01GB    |
+----------------+------------+------------+
| school         | 1.54GB     | 2.28GB     |
+----------------+------------+------------+
| gitlab         | 4.50GB     | 5.81GB     |
+----------------+------------+------------+
| haproxy        | 910.59MB   | 1.26GB     |
+----------------+------------+------------+
| keycloak       | 1.36GB     | 1.87GB     |
+----------------+------------+------------+
| nextcloudtest  | 541.10MB   | 847.96MB   |
+----------------+------------+------------+
| onetimesecret  | 4.37GB     | 4.68GB     |
+----------------+------------+------------+
| gateway        | 3.23GB     | 9.29GB     |
+----------------+------------+------------+
| video1         | 1.02GB     | 1.60GB     |
+----------------+------------+------------+
| video2         | 1.12GB     | 1.78GB     |
+----------------+------------+------------+

I checked with ncdu and found that there a video files in a download directory on the webserver that a 3x of the size on the local server.

Something is really strange in here. Any hints what is going wrong?

Edit: Sorry didn’t read the lxd vesion

Have you migrated the source lxd snap some point from apt to snap package?