ASDF's process for constructing a build plan from partial-order dependencies is (unless Faré changed something when I wasn't looking) non-deterministic.
does it mean that it's actively randomized?
i remember in our busier days (well before ASDF3) we used to have load order anomalies that seemed like they depended on filesystem order or something external to lisp.
the symptom was that on my dev machine stuff compiled/loaded fine reproducibly, then when the recorded changes got pulled to the live server it suddenly failed to compile/load due to a dependency that was not explicitly added to the .asd file.
i don't really remember if it was just partial reloading, or clean recompile/reload, but i think it was the former, because it was rather baffling and the former wouldn't be too baffling.
if the ordering is actually not deterministic (as opposed to merely unspecified), then maybe it's a good idea to actively randomize lists before dependency constraints are applied?