My csv file was created with an old version of Excel for Mac which apparently has issues with creating proper line endings.
On a Mac, Excel produces csv files with the wrong line endings, which causes problems for git (amongst other things).This issue plagues at least Excel 2008 and 2011, and possibly other versions. Basically, saving a file as comma separated values (csv) uses a carriage return \r rather than a line feed \n as a newline. Way back before OS X, this was actually the correct Mac file ending, but after the move to be more unix-y, the correct line ending should be \n. Given that nothing has used this as the proper line endings for over a decade, this is a bug. https://nicercode.github.io/blog/2013-04-30-excel-and-line-endings/
Creating a new file blank file and pasting in the contents fixed it.