[OnePlus 8 Questions & Answers] jiml8:
This started a couple of days ago, and I have now mitigated it with a couple of firewall rules on the VPN gateway, as well as shutting down the dhcpcd server on that server (which I don't need anyway, and which probably should have been stopped long ago).

My LAN has a raspberry pi 4 running their debian firmware that is configured as a VPN gateway. It connects my LAN via ProtonVPN to the internet. This gateway is set up with a static IP address (192.168.2.49) on the LAN, and is configured to use another RPI on my LAN to get its DNS (192.168.2.50).

My one month old running OOS 11 OnePlus8 is rooted with magisk, and I have blocked most of the google stuff from the internet using afwall, and suspended non-essential system services using greenify. When connected to my LAN, the phone has a static IP address (192.168.2.71), has its gateway set to the VPN gateway (192.168.2.49), and its DNS to my local rpi DNS (192.168.2.50).

DHCP on my LAN is provided by my router (192.168.2.1).

WIFI on my LAN is provided by an enterprise-grade tp-link hotspot.

Starting a few days ago, for reasons mysterious, when the phone connects to the LAN, the VPN gateway would promptly go offline. Because I run it headless, I would be forced to reboot it - which made diagnosis a bit of a pain. Finally, I found a log entry on the VPN gateway that informed me that my OnePlus was trying to claim the ip address of the VPN gateway as its own (192.168.2.49) in spite of being set to use 192.168.2.71. This duplicate IP was causing dhcpcd on the VPN gateway to immediately take down its eth0 interface. This would break ALL connectivity because I have wifi on that RPI disabled.

Prior to this problem involving the OnePlus, that RPI had been up continuously for over 400 days, so it should certainly be considered to be reliable at the job it does and almost certainly the problem is with the OnePlus.

So, for some reason the OnePlus is trying to assert its assigned gateway address as its IP rather than the 192.168.2.71 that is set, at least in some packet that it uses to announce itself; once it is connected it works properly (which means the right IP address is being used).

I have deleted, then re-created the wifi connection profile and doing that did not cause the problem to go away.

I have another RPI VPN gateway on my IOT VLAN (192.168.24.0/24). No DHCP is available on the VLAN (a security measure), and I do have a profile for the phone that allows it to connect to the VLAN. It works without issue there, but then dhcpcd has been and remains shut down on that RPI. I suppose I could start dhcpcd on that server and see if the phone then breaks it too. I won't do this unless there is some merit to doing so...if it would help find the basic problem.

As I say, shutting down dhcpcd and blocking all dhcp traffic to/from the LAN VPN gateway mitigated the problem. But that the problem could occur at all says something is wrong, and I'm pretty sure it isn't a problem on my network.

This seems most likely to be a bug in OnePlus firmware, though why it would manifest after a month is a mystery to me. Does anyone have any insight? Or does anyone have any suggestions for another place on XDA where this post might more appropriately be placed?