If I follow correctly, timing is reported as being met when optDesign -postRoute finishes, however when the design is re-timed after optimization has finished, timing is reported as *not* being met? If that's the case- I'd wonder how timing is being reported. Is it with "timeDesign -postRoute? If so then it seems like a bug. If not, try timeDesign -postRoute and let us know what you find.
Either way, it sounds like you have a situation that would we could use to improve the software and/or learn from as users of the tool.