I have exacly the same issue. I set SIM 1 as the preferred one for data & SMS, and to be asked each time for calls. I also turn off data on SIM 2. Since the update to Android Pie, the settings will automatically change to set SIM 2 as the preferred one for everything, turning data on SIM 2 back on. Every time that happens, I get the pop up to apply nework operator settings and to reboot, with no choice but to comply, as there's only an 'Ok' button. After rebooting, I have to change the settings and to reboot again. I have a bit more luck than the OP, as it will happen every two days and not several times a day, but it is highly annoying and makes me seriously consider downgrading back to Oreo.
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