Incorrect INPC021 when using setter method

[jnm2]

class C : ViewModel
{
    private string? p;

    public string? P
    {
        get => p;
        /* INPC021 Setter should set backing field.
        ↓ */
        set => SetP(value);
    }

    private void SetP(string? value)
    {
        Set(ref p, value, nameof(P));
    }
}

The motivation of using such a method is to give it a second parameter.