Update: I just noticed something weird. Windows keep resetting GP "Turn off MS Defender Antivirus" after every reboot. It resets to "Not Configured." That's the real issue.
First things first. Are you even using Windows 11 PRO? Because GPOs on Windows Home are not supported by Microsoft.
That setting will only work if your system is enterprise or government managed with a volume Microsoft license.
There are many GPOs that do not work if not managed. Microsoft does this on purpose. For one Microsoft does not want consumers to be able to disable any protections, particularly Microsoft Defender. Microsoft does not go on record publicly about the decisions it makes with regards to Defender nor Group Policy.
Perform internet search "Why does Group Policy not work?" and read through a bunch of them.
Windows is a generic operating system that is meant for:
1. Enterprises and Government (Home use is just an afterthought)
2. Managed / Domain-joined and fully managed by an enterprise level admin using AD/AAD, InTune, SCCM, etc (Unmanaged home use is just an afterthought)
To further complicate matters there are many instances of GPOs not working at the enterprise/government level. You can spend months researching GPOs on Windows and in the end you will likely be no further than you are today. Just leave it be and move on because your system is not managed. This is how the world of Microsoft works. Always has. Always will.