One thing that's hard about rewriting code is that the original code was transformed incrementally over time in response to real world issues only found in production.

The code gets silently encumbered with those lessons, and unless they are documented, there's a lot of hidden work that needs to be done before you actually reach parity.

TFA is a good list of this exact sort of thing.

Before you call people amateur for it, also consider it's one of the most softwarey things about writing software. It was bound to happen unless coreutils had really good technical docs and included tests for these cases that they ignored.