System partition locked to read-only in Android 10

I have a Xiaomi Mi A2 running Android Ice Cold Project (AICP) nightly build 15.0 (Android 10). I don't know what I did, but after installing a custom ROM, I noticed that my system partition cannot be mounted as read/write. I tried 1. mount -o rw,remount /system command on a terminal emulator, ADB shell, and TWRP terminal. The command shows no feedback showing failure (maybe I succeeded?) 2. Mount it using the built-in TWRP mount function. The system check box can be checked out (it doesn't retain its check mark on reboot) Some apps actually work according to sources and reviews. Some say (Failure to mount..), some say (System mounted as rw), and some don't have feedback. I don't know what to do. I noticed this when I installed a custom ROM. I tried using another custom ROM but same thing. Also, reformatting doesn't help. I don't know if this occurs on the stock ROM.