Contacts Merged With Another iCloud Account

Several months ago I broke my iPhone X and needed to use an old iPhone 6 my mom had lying around while my replacement phone was being shipped. In order to keep my number and contacts, I popped my Sim card out of my old phone and put it in the temporary one that used to be my mom's. Suddenly everything, and I mean everything, in both of our iCloud accounts merged, without it asking. We were even getting text messages meant for the other person even though I was using my own Sim card and phone number. I managed to log out of iCloud on the temporary phone, but was unable to do so on my broken phone because it wouldn't even turn on. That seemed to fix the issue of receiving each other's text messages, but ever since, our Contacts will randomly re-merge, without either of us doing anything to make that happen. It's like our iCloud accounts are somehow permanently linked, even though I logged out of iCloud on the temporary device and have never signed in to her account on my new device. It happened again today and suddenly I have 100+ contacts in my phone for people my mom knows and I don't. I have no idea how they randomly reappeared in my phone and don't want them there.

Is the only way around this to create a whole new icloud account?? I'm afraid if I un-sync the contacts in Settings, that I'll lose some of the ones I want to keep and won't know they're gone until I need them.

Why does this keep randomly happening? Any way to permanently fix this without a whole new iCloud account??