I believe I have the solution, I received the following response from Shopify:
I am sending you this follow up email as promised in regards to your concern earlier about the automations for abandoned cart and abandoned checkout not being sent to the customers. Upon coordinating with our Support team, we were able to see that the reasons why these automations are not being trigger are because of the products not being quantity tracked and the sender email being unauthenticated. In order yo resolve this then, you need to authenticate the sender email that you are using by going to your admin settings > Notifications > Authenticate your email and then copy all the records that are shown and paste those records in your domain’s DNS settings. After all the DNS records were pasted on the DNS settings of your domain, you need to go back to the Notifications page and then finally authenticate the email.
Additionally, please ensure that the quantity of your products are also being tracked since one of the conditions of the automations is for the product to be in stock. That being said, the products’ quantity needs to be tracked so that the automation can detect if the product in the abandoned cart or checkout is in stock. You can follow the steps on the screenshot below so you can track the quantity of your products.