QST: Why merge() output index value to a column?

[ericpai]

Research

• I have searched the [pandas] tag on StackOverflow for similar questions.

• I have asked my usage related question on StackOverflow.

Link to question on StackOverflow

https://stackoverflow.com/questions/77094543/the-rule-of-the-output-columns-in-pandas-merge

Question about pandas

No response

[rhshadrach]

This looks buggy to me. Further investigations are welcome!