A friend of mine has an OPPO phone which is running their version of Android called colorOS. He's on colorOS v11.1, based on Android 11. His phone does the same thing. We were trying to configure it with his router and Pi-hole and no matter what we did the phone always adds an additional DNS entry for 8.8.8.8. There's no warning that it chooses to do this nor seemingly any way to disable the behaviour.
Interestingly OPPO and OnePlus are very close so this probably explains the same behaviour on both brands.