Recurring issue - files not available

Hi all. I’m running Nextcloud 19 with groupfolders enabled. Every so often, I get the message that “files are not available”. Normal dirty fix is, move the whole folder out of the data directory, then move it back in, change ownership to Apache and voila, no more issues. But today I decided to see if I can get to the bottom of this.

I have my data folder living on an NFS, but it mounted as a “local” drive in the OS. In other words I am not using external storage mapped in the Nextcloud UI itself.
I got the following excerpts from the logs:

Sabre\DAV\Exception\ServiceUnavailable: Could not open file
failed to open stream: Permission denied at /var/www/html/nextcloud/lib/private/Files/Storage/Local.php#352

When I checked the permissions on the files in question, all permissions were correct and the same as the permissions for other working files. The permissions for all the parent folders up to the point of the broken files, were also correct.

So what could be the cause of this? I have even incorporated audit_log to see if someone is perhaps deleting the files or not, but that does not tell me anything. In fact, it tells me when someone accessed the file (even though it spewed out 503 error code on the front-end).

Nextcloud version: 19.0.1
PHP version: 7.3.24
MariaDB version: 10.4.15

No one have seen this issue before?