SharePoint: site Quota threshold exceeded, but no notifications received

I am looking at setting site quotas across our tenancy, to control storage growth by prompting users to think about what they're uploading, and whether old files can be deleted.

I set a low quota (1GB) on a test site, with "Allow notifications" checked, and a threshold set. I then stared copying files to the site. The site is currently shown to be at 87% of its quota in SP Admin centre, and above 98% according to the storage metrics, with the threshold set at 80%. If I go to Site Settings, I see a very narrow banner informing me that the site is out of storage space. This is not displayed anywhere else, though, I (as site owner and primary site admin) haven't received any email notifications, nothing is shown outside the settings pages - I can upload another file with no warning. It is now over 72 hours since I started the testing.

If site owners need to go to a settings page to see a warning, it is unlikely to achieve the desired effect and will be disruptive to users when sites do unexpectedly run out of space. Is there something else we can (or need to) do to enable notifications to site owners?