I am using Creo Parametric Release 6.0 and Datecode188.8.131.52
A user has a sheetmetal part (80002680) that has been mirrored (80002681). When trying to create a flat pattern for the mirrored part, it was discovered that the mirrored part is not sheetmetal part and therefore cannot be unbent. When editing the definition mirrored merge in an attempt to correct the issue, the system exits prematurely. See the attached models, trial file, and log.
Here are the errors that I faced
See Traceback Log
I had a coworker with the same problem. We simply converted the mirrored part to sheetmetal and all is good.
I believe the issue is he started with a standard start part (not sheetmetal) and created the mirrored part as geometery only.
As you can see in little movie (end of post), the behavior you're describing here is not reproducible while starting from a simple sheetmetal model in 184.108.40.206 (your version). The Flat Pattern feature in the mirrored sheetmetal part is produced as expected, whatever if Mirror Part was "saved as" starting from the original Sheetmetal model using Geometry only or Geometry with features logic.