Every time my system locks or logs out, my user account name gets changed. My last name has two capital letters, like John McSmith. Every time it gets logged out, it is changed to only capitalize the third letter of the last name, like John mcSmith. I find this odd, so I go in to System Settings -> My Account and update it. It appears to save, but then gets rewritten the next time I'm logged out.
That's odd! Let's try to do this from terminal. Please open a terminal by pressing ⌘+T and execute the following command,
sudo usermod -c "[the name you want]" [your username]
For example, lets say your username is "david" and your name is "John McSmith",
sudo usermod -c "John McSmith" david
It will ask for user password, once done you can see the changes in User Account Settings. Please log out now and see if your name is changed again. I hope it won't.