Interesting… I remember posting this question on Slack, where a partner provided the following link:
However, just now, on my dev store, I add two items to the cart and they appeared in the order I added them in (item #1 first, item #2 second). I am testing an app that uses Cart Transform, so the items in the cart were transformed into a bundle (as desired). On checkout, the items in the bundle were in the reverse order from how they were added to the cart (#2 first, #1 second).
Head-scratching… 