Polars made the mistake of not maintaining row order for all operations, via the False-by-default argument of maintain_order. This is basically the billion-dollar null mistake for data frames.
Yeah that really should have been default. Very big footgun, especially when preserving ordering is default in pandas, numpy, etc. And especially when there is no ingrained index concept in polars, people might very well forget that one needs to have some natural keys and not rely on ordering. One needs to bring more of an SQL mindset.