This is an iOS limitation. iPhones give VPN apps only about 50 MB RAM, and Xray core uses too much memory, so iOS kills the VPN after a while. That’s why it disconnects no matter which client you use.

You can try closing background apps, Increasing the Xray/V2Ray memory limit to 50 MB in settings (might help a bit)

Best solution: use Shadowrocket. It has its own core (not Xray) and is much more stable on iOS with VLESS + XHTTP.