Indeed, looks like there is a backlog of expired snapshots:
lxc operation list
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| ID | TYPE | DESCRIPTION | STATUS | CANCELABLE | CREATED | LOCATION |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 0c7d4bea-1cb2-4453-ace7-4392ce3103e1 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 19:41 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 0c14dedb-e34c-4745-9a7d-8cbb8f63b993 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 18:54 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 0db0cc9b-9488-4d4d-a1b6-df67527c795d | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 17:24 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 0e1e77c1-2d8a-40e8-bca7-7459d7977c73 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 16:59 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 0fa0967d-4a09-4036-ab8c-f74ef11b9883 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 14:53 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1aff389c-2b84-4d05-abf0-7290716848f5 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 14:24 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1b4c45eb-f2a8-424c-b0bb-378110a1c97a | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 20:11 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1b150ab9-a137-492d-abf3-9f9a4c54fdd7 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 16:44 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1bda3d26-50c5-48ce-b878-a922f939d65c | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 14:11 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1d587f49-c7bb-4412-b342-5fece5154698 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 16:57 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
| 1d814005-6177-46bf-906f-fd4a387f1cd3 | TASK | Cleaning up expired instance snapshots | RUNNING | NO | 2021/11/17 15:41 UTC | server1 |
+--------------------------------------+------+----------------------------------------+---------+------------+----------------------+----------+
...
370 tasks altogether!