Random musings: it’s not your cell ISP seeing “that weird IPv6 stuff” and disconnecting you as some sort of stupid “precaution”? Could you try with another SIM that’s on a different cell ISP to see if it still happens? v6 local disco shouldn’t be going outside of your local network and when on WiFi, there shouldn’t be any cell data, but this seems such a bizarre problem that it’s worth a try.
I got a 3 XL but I am not on that network, and I see no issues.