Hi all,
I have the solution for this.
Customers are being unsubscribed after purchase because when doing a purchase, there is a box on checkout which says “would you like to subscribe to our newsletter?” Or something similar. When this is left unchecked by the customer, and the customer inputs the same email when purchasing (which they previously used to subscribe), it will unsubscribe them from the email list.
Now the best way to get around this is to make that check box automatically ticked through the Shopify settings on the checkout page. This way customers don’t have to check the box, it’s already done, and thus your subscribers will still stay subscribed after purchasing.