Why are customers' carts are emptying when they add more than one item?

[Mage23]

This has been tested on multiple OS and both laptop and mobile. Everything is updated. Checking out one item works fine, but soon as someone returns to the store to add more, the previous item is always removed and customers can't order everything in one go as they should be able to. 

We are using the Dawn theme

Sales channels:
Online Store
Google and YouTube
Inbox
Point of Sale
Shop
Spotify for Artists

Apps:
Printify
Search and Discovery

Thanks for your help

[Mage23]

Updating from Dawn 15.0 to 15.2 and changing the color of the add to cart button resolved the issue, somewhat, anyway. The problem still remains if a shopper hits "buy with shopify" instead of add to cart, and then hits the back button or the link to our store from the cart. Can we fix that?