You say that the VPN is always on, but wasn't there a post on HN some time ago that showed that on iOS VPN is not 100% used for all data? Might have been this one: https://www.michaelhorowitz.com/VPNs.on.iOS.are.scam.php
So potentially your real IP was leaked otherwise, Google became aware of that IP and geolocated you, without Maps data?