How to block OTA upgrade to Android 13

[InteRadek]

After my phone was OTA upgraded to Android 13, I started to have issues with Bluetooth. I decided to downgrade to Android 12. After downgrade I disabled automatic upgrades (screenshot below) before I connected the phone to the Internet.
I spent several hours configuring my phone from scratch. For 5 days I was happy with Android 12, there were no issues and there were no notifications about upgrade being downloaded. The phone was also rebooted several times.
On day 6 I put my phone to my pocket. 10 minutes later I reach for my phone and realize it is powered off. When I turned it on, it was upgraded to Android 13.
There were no notifications about upgrade being downloaded or warnings about pending upgrade.
Why my phone was OTA upgraded to Android 13 despite disabling the automatic upgrades? How to prevent it from being upgraded in the future after downgrading back to Android 12?

[InteRadek]

Anybody downgraded to Android 12 and didn't get automatically upgraded back to Android 13 for at least a week?

[InteRadek]

There is a way to temporarily stop OTA upgrade. In Applications section, when Show system apps option is selected, on the list there is System update app (DM Client). When the app is Force Stop, it will cease to download OTA update. Unfortunately after few hours it will be started again and will start to download the OTA update again.

So there is no real permanent way to block OTA upgrade except perhaps for getting root.

[InteRadek]

In addition, in Developer options there is an option to disable Automatic system updates. Disabling this option is also ignored and won't prevent from being forced to upgrade to Android 13.

[InteRadek · Accepted solution]

I found a working way to block OTA upgrade. I use no-root firewall (NetGuard) to block Internet access for System update (com.asus.dm) app. To see the app on the list, select Manage system apps in Advanced options.