Basic Light rotation not working in viewport/preview, yet works in standalone

Development Rendering
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2

I think your object iterator is causing a problem, you can try this code :
This code is in the BeginPlay

TArray<AActor*> FoundLights;
	UGameplayStatics::GetAllActorsOfClass(GetWorld(), ADirectionalLight::StaticClass(), FoundLights);

	for (AActor* TActor : FoundLights)
	{
		if (TActor->GetName().Equals("SunLight"))
		{
			MySunLight = Cast<ADirectionalLight>(FoundLights[0]);
			break;
		}
	}

and this one in your tick :

if (IsValid(MySunLight))
	{
		FRotator MyCurrentRot = MySunLight->GetActorRotation();
		MySunLight->SetActorRelativeRotation(CurrentSunRotation);
	}