Since we have no idea what your data contains or what output you actually get from the sort, we can't really help you.
So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. Start here: pdb — The Python Debugger — Python 3.11.1 documentation
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!