My thought process, if someone handed me this code and asked me to fix it, would be that they probably didn’t expect df[‘index_value’] to hold df.index

Just because, well, how’d the code get into this state? ‘index_value’ must have been a column that held something, having it just be equal to df.index seems unlikely because as you mention that’s always been available. I should probably check the change history to figure out when ‘index_value’ was removed. Or ask the person about what that column meant, but we can’t do that if we want to obey the prompt.

The model (and you) have inferred completely without context that index_value is meant to somehow map to the dataframe index. What if this is raw .csv data from another system. I work with .csv files from financial indices - index_value (or sometimes index_level) confers completely different meaning in this case.