Wrong indentation after the fourth reference

@DonaldArseneau, I think that's the reason. Thank you.

@incollection{Chaum1991,
  doi = {10.1007/3-540-46877-3_41},
  url = {https://doi.org/10.1007/3-540-46877-3_41},
  year = {1991},
  publisher = {Springer Berlin Heidelberg},
  pages = {458--464},
  author = {David Chaum},
  title = {Zero-Knowledge Undeniable Signatures (extended abstract)},
  booktitle = {Advances in Cryptology {textemdash} {EUROCRYPT} '90}
}

As you can see the URL contains an underscore _, and it was rendered as a subscript. Almost all of my references contain the _, that's also why no matter how I move it around, I still have the problem.

Change it to _ solve the problem.