The sorting icon is not displayed when the column is clicked to sort by asc/desc.
See attached image: (The index column is sort by asc order by the down arrow is not displayed)
This broke due to the upgrade from DataTables from 1.10.20 to 1.10.25. I have confirmed that version 1.13.2 in master branch does not have this problem.
For reference, in 1.13.2, it looks like the following: