Data is here: https://gist.github.com/anonymous/2a7beb4871b25ae8f8b3

This results in line 7 being sorted incorrectly: sort -t , -k 1n < weird.csv

This produced the expected results: cut -f , -d 1-3 < weird.csv | sort -t , -k 1n

Using 'g' instead of 'n' also produces the expected results, but I'm not clear on what the difference is between 'g' and 'n'.

Tested with sort 8.21 on Slackware64-current.