The sort utility defaults to folding lower case to upper case. This is only supposed to be enabled when the -f flag is given, and there is no means to turn it off. This in turn breaks the use of ctags to make tags files for vim to use since vim requires sorted tag files that don't fold lower and upper case letters when sorting.
It's not a bug, it's a feature. sort now respects locales. What you want to do is setting LC_ALL=C before using sort.