Hi -
A little more detail for you based on my experience. The LIFO behavior you refer to is how it has always worked, and how it SHOULD work. I confirmed as much with the Shopify team.
What was happening to me, however, over a period of weeks is that the LIFO would suddently switch to FIFO, stay that way for a few days, and then switch back. It was inconsistent.
The “caching” client side issue was only related to whether or not the session had a cookie already associated with a cart. So, for example, I could test using my main browser that I normally am on my shop with. The behavior would be consistent, but then I’d open a new private session and see that the behavior for new carts (cart as a proper object on the Shopify server side, like a session) would have flipped.
Ever since I got through to the right team, and they investigated (took weeks) the issue was mitigated, the behavior has stayed normal - that being, the ajax cart add will manifest as LIFO when you go to the cart or order. So if that’s what you are seeing, that would be correct. Until Shopify arbitrarily changes it.