Do you have an Apple Watch by chance? If so, see which undesired System Services are enabled on the Watch, then use your iPhone’s Settings to turn them off by toggling on and then off (since I’m quite sure they aren’t configurable on the watch itself). This finally fixed the System Services resetting bug for me before I ditched my Apple Watch. Now those toggles—including the oft-dreaded Significant Locations—stay where they should.
Sorry if I didn’t articulate clearly enough, but I’d just give it a play and see if you and others can maybe remedy the situation for yourselves. And if you don’t have an Apple Watch I’m not sure what to tell you but I’d still maybe try playing around with the Location settings a bit. Good luck.
Fun and perhaps undesired fact: The only System Services I have enabled now are Emergency Calls & SOS, Wi-Fi Calling, System Personalization, and Setting Time Zone—the barest of essentials in case I need first-responders to know where I am and so that Night Shift automatically turns on and off with my local sunrise/sunset. No, I no longer use Find My…probably effects a good bit of battery drain and because I have AppleCare+ with Theft & Loss I’ll take the risk of being a responsible adult with my possessions.