Same ISP, WhatIsMyIPAddress shows IPv6 on Android, but not on Windows

[libove]

I have a Windows 10 computer and an Android phone side by side, connected to the same Wi-Fi of the same ISP router.
The WhatIsMyIPAddress.com website shows an IPv6 address for the Android device which properly identifies the ISP (even when that Android device has a VPN connected), whereas WhatIsMyIPAddress shows the Windows' system's IPv4 address (which changes to the VPN endpoint when the VPN is connected).
IPCONFIG/ALL on the Windows machine DOES show IPv6 configured, just like on the Android phone.
So, why does WhatIsMyIPAddress show IPv6 (and cut through the VPN's intent to hide behind a foreign IPv4 address) on my Android phone, while WhatIsMyIPAddress seems oblivious to IPv6 on the Windows computer?

Relatedly, it seems that WhatIsMyIPAddress should try to identify and display both the client's IPv4 and IPv6 addresses, for completeness.

[Chrispcritters]

The site shows the IP address where the request comes from.

When we're showing your actual IPv6 address when you are using a VPN it's because the IPv6 traffic is not being routed through your VPN -- basically over IPv6 you're 100% exposed. This is called "IPv6 leaking." Your VPN provider should have instructions available on how to prevent it from happening. on your mobile device.

We're still beta testing our dual stack check and will make it available on the homepage once we're happy with the methodology, display, and stability.

[libove]

Could you post, or private message, the URL of the beta-test dual stack check, please? Thanks.
But, in any case, the question really is: Since BOTH of these devices - one Android and the other Windows - have dual stacks, run the same VPN client (PureVPN), and are on the same Wi-Fi network behind the same ISP router, why does one of them show IPv4 and the other IPv6 on WhatIsMyIPAddress? Shouldn't both behave equally?

[amosb]

Why doesn't "WhatIsMyIPAddress.com" return IPv4 addresses and is there a way to obtain them?

[Chrispcritters]

We're in the final stages of testing the mechanism that will display both the IPv4 address and the IPv6 address, at the same time, on the homepage, regardless of whether the initial connection to our site came via IPv4 or IPv6.

For the time being, you can visit https://whatismyipaddress.com/ds-check