Why does my SystemD service not restart on reboots? - amazon-ec2

Question

I am building an AMI using Packer, which has a custom SystemD unit configured. The AMI is then deployed to an EC2. The issue is that if the EC2 reboots, then the unit is not restarted.
Here is my unit:
[Unit]
Description=My service
After=network.target
StartLimitIntervalSec=0
StartLimitAction=reboot
[Service]
Type=simple
Restart=always
RestartSec=30
User=ubuntu
ExecStart=/opt/app/app
[Install]
WantedBy=multi-user.target
And here is my Packer configuration:
{
"variables": {
"aws_access_key": "{{env `AWS_ACCESS_KEY_ID`}}",
"aws_secret_key": "{{env `AWS_SECRET_ACCESS_KEY`}}",
"region": "{{env `AWS_REGION`}}"
},
"builders": [
{
"access_key": "{{user `aws_access_key`}}",
"ami_name": "my-app-{{timestamp}}",
"instance_type": "t2.micro",
"region": "{{user `region`}}",
"secret_key": "{{user `aws_secret_key`}}",
"source_ami_filter": {
"filters": {
"virtualization-type": "hvm",
"name": "ubuntu/images/*ubuntu-bionic-18.04-amd64-server-*",
"root-device-type": "ebs"
},
"owners": [ "099720109477" ],
"most_recent": true
},
"ssh_username": "ubuntu",
"type": "amazon-ebs"
}
],
"provisioners": [
{
"type": "shell",
"script": "{{template_dir}}/provision.sh"
},
{
"type": "file",
"source": "{{template_dir}}/files/app.service",
"destination": "/tmp/upload/etc/systemd/system/app.service"
},
{
"type": "file",
"source": "{{template_dir}}/../bin/Release/netcoreapp3.1/",
"destination": "/tmp/upload/opt/app"
},
{
"type": "shell",
"inline": [
"sudo rsync -a /tmp/upload/ /",
"cd /opt/app",
"sudo systemctl daemon-reload",
"sudo systemctl enable app.service"
]
}
]
}
Strangely, if I SSH into the running EC2 and enable the service, then it does restart after a reboot.
sudo systemctl enable app.service
sudo reboot
This makes me think I am not creating the AMI correctly, but in my Packer configuration I do enable the service!
Why does my AMI not have my SystemD unit enabled?