All cursors except the default 'arrow' cursor are using the wrong theme (the default X11 theme I think, rather than 'DMZ-White'). Not just in the editor, but in all dialogs as well. If I go to my system settings and re-apply the cursor theme (which presumably sends a 'cursor theme changed' message to applications), it's fixed. But the bug reappears as soon as I restart Qt Creator.
This never happened in any previous (Qt4-based) version that I've used, so I think it's a Qt5 bug. I haven't tested any other Qt5 applications, so I'm not sure.