vacuum_storage report missing files from storage

On debusine.freexian.com where vacuum_storage did complete regularly, logs regularly show errors like those:

May 29 02:00:01 hestia systemd[1]: Starting debusine-server-vacuum-storage.service - Regular Debusine storage maintenance...
May 29 02:00:01 hestia debusine-admin[922001]: System check identified no issues (0 silenced).
May 29 02:00:02 hestia debusine-admin[922016]: Checking orphan files from stores
May 29 02:03:14 hestia debusine-admin[922016]: Checking empty directories
May 29 02:03:39 hestia debusine-admin[922016]: Checking incomplete artifacts
May 29 02:03:39 hestia debusine-admin[922016]: Checking orphan files in upload directory
May 29 02:03:39 hestia debusine-admin[922016]: Checking missing files from stores
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/69/fe/45/69fe45f82dada01b3f8df2eef157b4de9ca2f681b408bcdfd21ca1ef5e9f7639-18870
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/46/dd/16/46dd16b9fbf089bb7501111951f0e3d904426a242cf1d560a4ce7e2e4956f9a1-12156
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/18/a0/28/18a028fec2fc1de8c24c85f53d6af2972f6688f2765a5e6202674da00179b573-21886
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/18/7b/ca/187bca6b8f3d8c4fa667a39d2dcb0f8e5def165c3d12c40900bc2c2e0e473cbc-18854
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/31/e5/6a/31e56a90578d4aac9262e2b3ca3ed5c9a5460d193002e242a9915a8bab21b7d6-15628
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/7b/5f/23/7b5f23d4281d736dc38e334f53138df5c1b58fc9041bf5c01768e158fad6794e-19304
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/1b/17/b5/1b17b5286f92c2cce1e9ede2f6ac5ced6891df5b0267fe3b01995a677a67ad73-45440
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/02/81/87/0281876addecb97a94ba13247592e3d5adddebd5bd32ee3bb79a1c657ff531ba-32224
May 29 02:05:39 hestia debusine-admin[922016]: File in FileStore Default but not in storage: /var/lib/debusine/server/store/1b/72/38/1b723855242b38c76e0042ff49d3cbbf9f7cf309ebb817fd978a4daa7df82623-19360
May 29 02:05:40 hestia debusine-admin[922016]: Checking missing files from upload
May 29 02:05:40 hestia systemd[1]: debusine-server-vacuum-storage.service: Main process exited, code=exited, status=1/FAILURE
May 29 02:05:40 hestia systemd[1]: debusine-server-vacuum-storage.service: Failed with result 'exit-code'.

The errors disappear on the next run of the command so it really seems like some concurrency issues where delete_expired removes files while vacuum_storage ensures they are all present.