Safetensors has an explicit ID tensor and I was ignoring it. Using row indices instead of the actual user-provided IDs. Upload a file with named vectors, query it back, get 0, 1, 2, 3 instead of the names you gave it.
The kind of bug that passes every test that doesn't specifically check ID correctness. So, all of them. My test fixtures were all synthetic with sequential IDs. Row index 0 happened to match ID "0". Everything looked fine until I tried it with real data where the IDs were UUIDs.
I need better test fixtures. Real ones with UUIDs and weird strings, not this sequential toy data that lets everything pass. Adding that to the list, which is getting long.