aSecondWriteReplacesTheFirstWhole

DurableStore.write "overwrites any previous value" — wholesale, not in place.

The second value is deliberately much shorter than the first, and that is the whole design of this property. A backend that writes the new bytes over the old ones without truncating leaves the first value's tail behind, and reading back gives a value that begins correctly and runs on into the previous one — invisible to any overwrite test whose two payloads are the same length, which is what the pre-existing per-backend tests mostly had.

The third write grows the value again, so a backend that truncated on the way down and then cannot grow back fails too.