Microsoft Graph Subscription Validation fails through Nginx

Question

When syncing calendars with an outside website, subscription validation for change notification first queries an endpoint before it responds. If it fails, we see this

Subscription validation request failed. Notification endpoint must respond with 200 OK to validation request.

This response is only possible if the access token is accepted. And it is. But Nginx log shows this

"POST /outlook_notification?validationToken=Validation%3a+Testing+client+application+reachability+for+subscription+Request-Id%3a+416879c6-172d-4d71-ae7e-21f3019babc2 HTTP/1.1" 405 166 "-" "-"

My server/program is proxied through Nginx. I can submit the same string above, myself, and it works. The id in the above string is returned as text, per instructions. But not when called by Microsoft. 405 error.

I can create Outlook calendar events, update and delete them. But I would like to receive notifications for a full sync.

Question - Does this only work with IIS? The validation POST doesn't even enter my server. Nginx rejects it. Why?