Have you noticed, when you log in with your uppercase username, it displays it in lower-case in the navigation bar?
I take some time to analyze this behavior and found the Theme itself is applying the lower casing with CSS.
.t-NavigationBar-item.has-username .t-Button-label {
text-transform: lowercase;
}
To restore the original text, all you need to do is head over to:
Shared Components > Navigation Bar List > Navigation Bar > &APP_USER.
Then scroll down to the User Defined attributes and you'll find this List Item CSS Class in slot #2.
Just remove the entry to make it blank
Then Apply Changes to the page & refresh your APEX Application
Voilá