Why does an Android app think I’m using a VPN when I’m not?

I ran into a strange problem today. An app keeps warning me that I’m using a VPN, even though I never turned one on and Android doesn’t show an active VPN connection.

What makes it confusing is that the warning isn’t always consistent. Sometimes the app works normally, and other times it immediately says that VPN or proxy use has been detected.

I’m trying to understand what the app is actually checking. Could it be the current IP address, Private DNS, an old VPN profile, an ad-blocking app, or just the network’s IP reputation?

If the problem is limited to one app, would per-app routing make any difference, or can the app still detect that Android has a VPN service active in the background?

I’m not trying to bypass the app’s rules. I just want to know how to identify whether this is a device-level detection issue or a false positive from the network…