Interesting. Your question made me realize there's some differences when I press CTRL+Space inside the arguments of ofDrawBitmapString("hi", 2, 2) and when I do that inside a test call to mouseDragged(4, 4, 4);
In the first case, the information about the arguments is only displayed if I put the cursor between "String" and "(". With mouseDragged I can place cursor before any of the arguments and the information is shown when pressing CTRL+Space. This does not work with ofDrawBitmapString.
There's some settings at Options > Text Editor > Completion (I guess you found those).
When I started with QtCreator and every time I updated it I had troubles with autocompletion. Now it's working for me, With QtCreator 4.2.1, clang 3.9.1, and a nightly build of OF.
If I type ofDrawBit, I can press down to select ofDrawBitmapString and then Enter to autocomplete.
What's at the top of your project qbs file? In mine:
import "/home/user/src/openFrameworks/libs/openFrameworksCompiled/project/qtcreator/ofApp.qbs" as ofApp
property string of_root: '/home/user/src/openFrameworks'