Comment by jorelfermin

Unfortunately (for your case) the ordering of group_by, join, and unique all run in parallel hash aggregation across the threads so the output order comes about by how it gets partitioned across cores. Which is why you can get different order of rows depending on the machine even when you have the same set of data and polars version (this has happened to me). To fix you can set maintain_order=True or probably better an explicit sort whereever you save or compare / diff the output.