While not the same issue, seems others are noticing some recent oddities around similar updates as well: https://community.shopify.com/c/Shopify-APIs-SDKs/Cart-API-Update-not-working-reliably/m-p/991838#M6...
Thanks for providing so much detail and sharing the screen recording. I think the approach you're using makes total sense. I've done some simple testing mimicking the behaviour you're attempting (without using the Fera Upsell app), and I'm not experiencing the issue. I'd be interested in tracking down the initiator of these requests.
When watching the video you shared, it appears that these are repeated GET requests to /cart.js. When debugging the issue, did you have a look at the stack trace on the Initiator column found in the Network tab of the Chrome developer tools? I was thinking the stack trace displayed when hovering over the initiator column could provide some clues to the client side code that's initiating these requests.
Look forward to hearing back from you.
Thanks for the response and glad the video was helpful, know replicating/troubleshooting code issues can be a bit tough without being in the same room.
Made a follow-up video here where I inspect the network call initiators and showcase how to replicate this issue in production (GTM code is not live, but show how I was running console JS to trigger it): https://www.loom.com/share/97b82c0c1e3b42f796197d5cd66a876b
However, if this still lacks some of the network call information you were looking for, let me know.