Can't overwrite .ini value

I’m trying to save the changes I make to the FullscreenMode key in GameUserSettings.ini

The changes are applied and the value is written into the GameUserSettings.ini but I can’t seem to overwrite the value once it’s set. In the DefaultGameUserSettings.ini I have FullscreenMode=2, in the GameUserSettings.ini I have FullscreenMode=0.

I’m following Rama’s Guide. This is what I got:

static FString GetIni(FString WhatIni)
{
	FString s = "";

	if (WhatIni == "GGameIni") s = GGameIni;
	if (WhatIni == "GScalabilityIni") s = GScalabilityIni;
	if (WhatIni == "GEngineIni") s = GEngineIni;
	if (WhatIni == "GGameUserSettingsIni") s = GGameUserSettingsIni;
	
	return s;
}

/***/
UFUNCTION(BlueprintCallable, Category = Config)
static void SaveStringToConfig (FString Section, FString RootDir, FString WhatText, FString WhatIni)
{
	if (!GConfig) return;
	//~~

	FString TargetIni = GetIni(WhatIni);

	//String
	GConfig->SetString(*Section, *RootDir, *WhatText, TargetIni);

	//ConfigCacheIni.h
	//void Flush( bool Read, const FString& Filename=TEXT("") );
	GConfig->Flush(false, TargetIni);
}

I also tried it with GConfig->SetInt(…) but it made no difference. Any idea what I’m doing wrong?