I use shopify and I see that even when I use proper syntax on the button like: www.mysite.com/discount/{% coupon_code 'UnengagedPurchased' %}, when I click on the button in the test email and click couple of pages on my web site, the code strips away and no discount code is automatically applied to the checkout.
What is the solution? Don’t want customers to copy paste the code and apply to the cart - this destroys the experience. The trouble is when ever you ask Klaviyo, they say, it is not our issue ask shopify.