You delete ten gigabytes of old backups, refresh the panel, and the quota bar has not moved. That is not a bug — it is a deliberate trade the daemon makes between an accurate number and not spending CPU walking your entire file tree every time the page loads.
Why the quota is not computed live
Computing a directory's total size means walking every file inside it and summing their sizes — the same work the du command does. Doing that on every panel page load, for every server on the node, at the same time, would be a meaningful and completely avoidable load on the very disk the server is trying to run from. Instead, the daemon runs that calculation on a periodic cycle — every 5 to 10 minutes — and the panel shows whatever the last cycle computed, rather than the true instantaneous figure.
Deleting a large file will not immediately show freed space, and — less obviously — uploading a large file will not immediately show it as used either. Neither is cause for concern; both catch up on the next cycle.
Restarting the container does not speed up the quota recalculation and is an unnecessary interruption if the actual goal is just to confirm the deleted files are gone. Waiting ten minutes and reloading the panel gets the same accurate number with none of the downtime.