In Azure "private networking", many components still have a public IP and public dns record associated with the hostname of the given service, which clients may try to connect to if they aren't set up right.
That IP will respond with a 403 error if they try to connect to it. So Azure is indirectly training people that 403 potentially IS a "network issue"... (like their laptop is not connected to VPN, or Private DNS isn't set up right, or traffic isn't being routed correctly or some such).
Yeah, I get that's just plain silly, but it's IAAS/SAAS magic cloud abstraction and that's just the way Microsoft does things.
You are not describing a network issue. You're sending requests that by design the origin servers refuse to authorize. This is basic HTTP.
https://datatracker.ietf.org/doc/html/rfc7231#page-59
The origin servers could also return 404 in this usecase, but 403 is more informative and easier to troubleshoot, because it means "yeah your request to this resource could be good but it's failing some precondition".