Right, if a use has manager role, they have preview capability, and if a user has both 'mod/quiz:preview' and 'mod/quiz:attempt' capability, then the 'preview' one wins, and they can only preview, and cannot make real attempts.
(The way these two capabilities interact is bad, and we really must fix it one day. It is logged as MDL-54875.)
(The way these two capabilities interact is bad, and we really must fix it one day. It is logged as MDL-54875.)