"your storage is almost full" with external storage

Nextcloud version (eg, 18.0.2): 18.0.6
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): Nginx
PHP version (eg, 7.1): 7.3

The issue you are facing:
The "“your storage is almost full” notice is appearing on 2 of 3 external drives.

Is this the first time you’ve seen this error? (Y/N):
No, it has been happening a long time, possibly from the initial setup.

Steps to replicate it:

  1. Log in and click one of the icons for an external storage.
  2. The notice appears in the top right corner.
  3. If you click the X and close it, it reappears when going to another folder in that storage.

We are using Nextcloud exclusively as a portal between remote workers and our internal NAS. We are not storing any data on the NextCloud internal drive, so the warnings are not needed.

Users come from LDAP/AD. They have unlimited quotas (which say unlimited in the User admin until they log in the first time, then it changes to 0).

The external storage is a NetApp connected to NFS shares. The 3 shares are each over 1TB.