CRM Plugin Did not Update All Fields

We're on CRM 2016 On-Premise. I have registered a plugin that gets triggered when two fields are being updated: fieldA and fieldB. The plugin will encrypt both fields, but for some reason it would only update the first field: fieldA

We registered the plugin as Post-Operation. We also registered a PostImage with both fieldA and fieldB as the filter. However for some reason it would only update fieldA and not fieldB.

Here's my code. I put a 'throw new InvalidPluginExecutionException' after the assignment of fieldB, but for some reason it is never reached. If I put InvalidPluginExecutionException BEFORE the assignment of fieldB (but still inside the if condition), then I would receive the error message.

I am not sure what is wrong with the assignment...

        string fieldA;
        string fieldB;

        var targetEntity = context.GetParameterCollection<Entity>(context.InputParameters, "Target");

        if (targetEntity == null)
            throw new InvalidPluginExecutionException(OperationStatus.Failed, "Target Entity cannot be null");

        var serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
        var service = serviceFactory.CreateOrganizationService(context.UserId);

        if (context.Depth == 1)
        {
            var postImage = context.PostEntityImages["PostImage"];

            if (postImage == null)
                throw new InvalidPluginExecutionException(OperationStatus.Failed, "Post Image is required");

            var account = context.GenerateCompositeEntity(targetEntity, postImage);

            if (targetEntity.Attributes.Contains("new_fieldA"))
            {
                fieldA = account.Attributes["new_fieldA"].ToString();
                targetEntity["new_fieldA"] = encrypt(fieldA);
            }

            if (targetEntity.Attributes.Contains("new_fieldB"))
            {
                fieldB = account.Attributes["new_fieldB"].ToString();
                throw new InvalidPluginExecutionException("test222"); //for some reason this message never shows up
                //targetEntity["new_fieldB"] = encrypt(fieldB);
                //targetEntity["new_fieldB"] = "hello";
            }

            service.Update(targetEntity);