IIS Site Randomly Returns 404 While App Pool Remains Started

Hello,

We have an IIS site running behind Azure Front Door that becomes unavailable every few days.

Symptoms:

Application Pool remains started

Users see the blue "Not Found" page from Azure Front Door

When accessing the site directly on the server during the failure, we get a 404 from IIS, not Front Door

The site does not recover on its own

Recovery requires an App Pool recycle or IIS restart, and sometimes a full server reboot because recycle or IIS restart does not help

What it is NOT:

No CPU, memory, disk, or network pressure

Event logs do not correlate with the outage

When it happens:

Random and unpredictable

Any insights would be appreciated.

From the server itself:

Please your support.