Webhook check fails once every so often on app with headers

I am getting a strange issue whereby maybe once every few weeks or so my app webhook gets disconnected due to failure to call.

I have two apps that communicate to the same endpoint on the same server, only one fails, the one which adds a custom header.

If I revalidate it will work for a time, maybe a few weeks again but will eventually fail again and block my webhooks.

Zoom does not give me a date or time or even status code that the webhook returned a failure so quite hard for me to track down.

What could be causing this?