We are pretty certain we resolved this based on feedback from another Shopify user. This is the information they sent me that resolved the problem for them, and we have our developer implement it. We are at a week, with 100% of our cart attributes feeding though again!
“there was a form field for the qty update that was causing an issue. It was always posting the note/date to the cart form, and I could see that, but it wasn’t being saved by Shopify.
When I disable the input field on qty it doesn’t get passed in the form, and then it seems to me like it’s now saving.
The user can still add/remove items with the + and -.”
Also, we found this infromation about a Shopify Ajax API change around the date all of us started having issues:
https://shopify.dev/changelog/changes-to-ajax-api-cart-token-format
Good luck!