From the Glacier Chat menu, tap Core Connection. Select a profile and toggle the Use Secure Core switch. When your connection is established a key icon will appear in your Android status bar.
In recent years, security researches have identified issues with both Android and iOS devices leaking data through the VPN connection.
"We researched the reported leak, and concluded that Android sends connectivity checks outside the VPN tunnel. It does this every time the device connects to a WiFi network, even when the Block connections without VPN setting is enabled." - Mullvad VPN
Google has responded;
"We have looked into the feature request you have reported and would like to inform you that this is working as intended. We do not think such an option would be understandable by most users, so we don't think there is a strong case for offering this."
In March of 2020 a security researcher discovered that in iOS version 13.3.1, the operating system does not close existing connections. (The issue also persists in the latest version, 13.4.) Most connections are short-lived and will eventually be re-established through the VPN tunnel on their own. However, some are long-lasting and can remain open for minutes to hours outside the VPN tunnel.
However, security researchers discovered the following technique to be almost as effective: