Your VPN isn't leaking. It's same with me. When we do speedtest via app, the speedtest app looks for connections details (mobile data or wifi). And in the speedtest(ookla) database they have records of all the tower locations and every tower has an unique id. So, when we do the speedtest with the app (not from website), the app detects user is using mobile data or wifi, if former is true, then based on the signal strength to the tower from that device, the app searches in the database about the tower info (belongs to which telecom operator) which the user is connected, then it shows the name of the operator.

This is a very interesting way of tracking users.

Even if you haven't enabled GPS, or mobile data or internet and your are using degoogled phone like GrapheneOS, if you have sim card there, you will still be tracked because the app (non open sourced apps) we use, as most(if not all) of them has this functionality built in, to detect users location based on signal strength to the tower.

So, stick with open sourced apps, and web apps if you can. And install the closed source apps on a different user profile and then turned that profile off when not in use.