Adding Fastly to a Heroku app does not forward to proper url

I currently have an Heroku app that is being served over a CDN. I have just added Fastly to my Heroku app as an addon and I am struggling to configure it properly. Here is my current configuration:

I have my domain registered on GoDaddy with the following CNAME configuration:

Host: www
Points to: my-site-herokuapp-com.global.ssl.fastly.net
TTL: Custom
Seconds: 600

Here is the GoDaddy forwarding configuration (not sure if this matters):

Forward to https://www.my-site.io/
(301 & forward only)

Here is my fastly configuration:

Domains:
my-site-herokuapp-com.global.ssl.fastly.net
*Provisioned for my-site via Heroku*

my-site.herokuapp.com
*Provisioned for my-site via Heroku*

www.my-site.io

And finally in Heroku in the domains configuration section of my app here are my settings:

Domain Name: my-site.io
DNS target: darwinian-kumquat-123456.herokudns.com

Domain Name: www.my-site.io
DNS target: serene-trout-123456.herokudns.com

Before I tried to make this change I had the CNAME pointing directly to: serene-trout-123456.herokudns.com an it was working fine. However after updating this value to the new Fastly value: my-site-herokuapp-com.global.ssl.fastly.net Fastly directs the site to my-site.herokuapp.com instead of just https://www.my-site.io. I followed the instructions listed here so not entirely sure where I have gone wrong. Any ideas?

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