VPC deployment error: "The new Subnets are not in the same Vpc as the existing subnet group"

My use case is that I want to deploy a Lambda in a vpc but also give it internet access.

I initially created a VPC with default parameters, i.e.

const vpc = new ec2.Vpc(this, "vpc", {})

After this was deployed, I decided to change the parameters of the VPC to the below.

const vpc = new ec2.Vpc(this, "vpc", {
      ipAddresses: ec2.IpAddresses.cidr('10.0.0.0/20'),
      natGateways: 1,
      maxAzs: 2,
      subnetConfiguration: [
        {
          name: 'private-subnet-1',
          subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS,
          cidrMask: 26
        },
        {
          name: 'public-subnet-1',
          subnetType: ec2.SubnetType.PUBLIC,
          cidrMask: 26
        }
      ]
    })

However, when I try to deploy the stack, I am getting the error: "The new Subnets are not in the same Vpc as the existing subnet group".

For context, I currently see 6 subnets in the VPC, 3 public & 3 private. I believe this makes sense if the default maxAZs of a VPC creation is 3.

A couple questions:

• Can you explain what the error means? I am confused because since the subnets are configured inside the VPC, how is it possible that changing the parameters would make new subnets outside the VPC?
• Is there a workaround for this error?
• Where can I view my 'subnet groups'? I can't find it on the AWS console

Thank you.

Edit: I realized this error is related to my Neptune. Here's the Neptune-related code:

// Get private Subnets
var privateSubnets = vpc.privateSubnets;

// Create Subnet selection for neptune.
const neptuneSubnets: ec2.SubnetSelection = { subnets: privateSubnets };

const neptune_cluster = new neptune.DatabaseCluster(this, 'neptune_cluster_id', {
      vpc: vpc,
      vpcSubnets: neptuneSubnets,
      instanceType: neptune.InstanceType.T3_MEDIUM,
      deletionProtection: false,
      clusterParameterGroup: neptuneClusterParameterGroup
    })