After not having a smooth release for v2.1, for the last 2 weeks I have been working on fixing the issue. I’m pleased to say the issue is now fixed but before I can 100% confirm that I need your help to test it further in case anything else was missed.
Version 2.1 was removed from WordPress.org so users cant rollback to it and I have reverted the stable version to v2.0.13 until this release is ready.
Here is the comparison log from v2.0.13
If you discover any bugs during the testing process, please let me know by logging a report in GitHub.
Thanks to all of the testers and contributors!
What’s New?
Synchronised Carts
One of the issues users reported with v2.1 was that the carts were no longer in sync. This is because CoCart runs it’s own session handler to support guest customers but for a registered customer their carts were not matching.
This was because on the web version of your store, it was still utilizing WooCommerce core session handler and reading data from their session table.
This has been corrected so now both your web and headless store use the same session handler. 
CoCarts session handler is 100% compatible with any WooCommerce extension or custom code you may have applied to your store as the handler was forked and tweaked to add support for guest customers. It’s practically the same… only better. 
Validating Products More
Variable products can be a hard time to manage and based on feedback, forgetting to pass or unable to pass the variation ID was one parameter users were failing to set correctly. Now you don’t have to as CoCart will get the variation ID based on the attributes you provide. 
Even the attributes get validation so encase you forgot to pass required attributes you will now be told which attributes you forgot. If a variable product is not valid it will not add to the cart.
Speaking of not adding products. Password protected products can not be added either now.
Full Changelog from v2.1 to v2.1.2
* NEW: Added support for guest customers. * NEW: Carts in session are stored in a new database table. * NEW: Carts are in sync across the web store and your headless store. * NEW: Added plugin details to **WooCommerce System Status**. * NEW: Added `uninstall.php` file to delete table and options. * NEW: Able to transfer a cart from your headless store to the web. * NEW: Added handlers to improve product validation and extended support for other product types. * NEW: Can now add items to cart using a products SKU ID. * NEW: When an item is updated, removed or restored... the cart totals are re-calculated. * NEW: Added option to logout customer. * NEW: Variable products are now validated and can find variation ID from attributes if not already set. * NEW: Prevent password protected products from being added to the cart. * Changed: Parameter used to set cart key. Previously `id` now `cart_key`. The `id` used as fallback if still used. * Removed: Parameter to re-calculate totals once item was updated. * Tweaked: Clear carts debug tool now clears saved carts as well. * Tweaked: Products that are no longer purchasable and are already in the cart are removed from the cart. * Tweaked: Stop redirect to getting started page if plugin was activated and was already installed before. * Tweaked: Prevent redirect to getting started page if multiple plugins activated at once. * Dev: Clear all carts stored in session via the Tools section of **WooCommerce System Status**. * Dev: Synchronize carts over to CoCart's session table in the database via the Tools section of **WooCommerce System Status**. * Dev: Cart expiration can be filtered if the default 30 days is not to your liking. * Dev: Generated customer ID can be filtered before storing cart in the database and creates a cookie on the customer's device. * Dev: Added filter `cocart_add_to_cart_validation` to allow plugin developers to pass their own validation before item is added to the cart. * Dev: Added filters to override the product name `cocart_product_name` and product title `cocart_product_title` when getting the cart contents. * Dev: Added filter `cocart_item_thumbnail_src` to override the source URL of the product thumbnail when getting the cart contents. * Dev: Added filter `cocart_add_to_cart_quantity` to override the quantity when adding an item. * Dev: Added filter `cocart_add_cart_item_data` so other plugins can pass cart item data when adding an item. * Dev: Added filters so the returned response messages can be changed. * Dev: Added conditional filter for returning a cart item. * Dev: Added hook `cocart_user_switched` to allow something to happen if a user has switched. * Dev: Added hook `cocart_load_cart` to manipulate the merged cart before it set in session. * Dev: Added hook `cocart_load_cart_override` to manipulate the overriding cart before it set in session. * Dev: Added hook `cocart_item_added_updated_in_cart` for when an item was added again but updated in cart. * Dev: Added a new class that handles logging errors. * Dev: Added filters to admin notices to extend the length of time they hide. * Dev: Added filter to override cookie check for authenticated users. * Tested: Compatible with WooCommerce v4.2.x > This update replaces WooCommerce core session handler with CoCart's. 100% compatible.